[ConstraintSystem] Detect and diagnose missing generic arguments

Introduce a fix to detect and diagnose situations when omitted
generic arguments couldn't be deduced by the solver based on
the enclosing context.

Example:

```swift
struct S<T> {
}

_ = S() // There is not enough context to deduce `T`
```

Resolves: rdar://problem/51203824

include/swift/AST/TypeRepr.h

@@ -365,6 +365,10 @@ public:
                                       ArrayRef<TypeRepr*> GenericArgs,
                                       SourceRange AngleBrackets);
 
+  unsigned getNumGenericArgs() const {
+    return Bits.GenericIdentTypeRepr.NumGenericArgs;
+  }
+
   ArrayRef<TypeRepr*> getGenericArgs() const {
     return {getTrailingObjects<TypeRepr*>(),
             Bits.GenericIdentTypeRepr.NumGenericArgs};

lib/Sema/CSDiagnostics.h

@@ -1244,14 +1244,11 @@ class MissingGenericArgumentsFailure final : public FailureDiagnostic {
 public:
   MissingGenericArgumentsFailure(Expr *root, ConstraintSystem &cs,
                                  TypeRepr *baseType,
-                                 ArrayRef<TypeVariableType *> missingParams,
+                                 ArrayRef<GenericTypeParamType *> missingParams,
                                  ConstraintLocator *locator)
       : FailureDiagnostic(root, cs, locator), BaseType(baseType) {
     assert(!missingParams.empty());
-    llvm::transform(missingParams, std::back_inserter(Parameters),
+    Parameters.append(missingParams.begin(), missingParams.end());
-                    [](const TypeVariableType *GP) {
-                      return GP->getImpl().getGenericParameter();
-                    });
   }
 
   SourceLoc getLoc() const;

lib/Sema/CSFix.cpp

@@ -540,3 +540,110 @@ CollectionElementContextualMismatch::create(ConstraintSystem &cs, Type srcType,
   return new (cs.getAllocator())
       CollectionElementContextualMismatch(cs, srcType, dstType, locator);
 }
+
+bool ExplicitlySpecifyGenericArguments::diagnose(Expr *root,
+                                                 bool asNote) const {
+  auto &cs = getConstraintSystem();
+
+  bool diagnosed = false;
+  for (const auto &paramsPerAnchor : Parameters) {
+    auto anchor = paramsPerAnchor.first;
+    ArrayRef<GenericTypeParamType *> missingParameters = paramsPerAnchor.second;
+    MissingGenericArgumentsFailure failure(root, cs,
+                                           anchor.dyn_cast<TypeRepr *>(),
+                                           missingParameters, getLocator());
+    diagnosed |= failure.diagnose(asNote);
+  }
+
+  return diagnosed;
+}
+
+ExplicitlySpecifyGenericArguments *ExplicitlySpecifyGenericArguments::create(
+    ConstraintSystem &cs, ArrayRef<GenericTypeParamType *> params,
+    ConstraintLocator *locator) {
+  llvm::SmallDenseMap<TypeRepr *, llvm::SmallVector<GenericTypeParamType *, 4>>
+      typeBasedParams;
+
+  if (findArgumentLocations(locator->getAnchor(), params,
+                            [&](TypeRepr *base, GenericTypeParamType *GP) {
+                              typeBasedParams[base].push_back(GP);
+                            }))
+    return new (cs.getAllocator())
+        ExplicitlySpecifyGenericArguments(cs, typeBasedParams, locator);
+
+  return new (cs.getAllocator())
+      ExplicitlySpecifyGenericArguments(cs, params, locator);
+}
+
+bool ExplicitlySpecifyGenericArguments::findArgumentLocations(
+    Expr *anchor, ArrayRef<GenericTypeParamType *> genericParams,
+    llvm::function_ref<void(TypeRepr *, GenericTypeParamType *)> callback) {
+  using Callback = llvm::function_ref<void(TypeRepr *, GenericTypeParamType *)>;
+
+  if (!anchor)
+    return false;
+
+  TypeLoc typeLoc;
+  if (auto *TE = dyn_cast<TypeExpr>(anchor))
+    typeLoc = TE->getTypeLoc();
+  else if (auto *ECE = dyn_cast<ExplicitCastExpr>(anchor))
+    typeLoc = ECE->getCastTypeLoc();
+
+  if (!typeLoc.hasLocation())
+    return false;
+
+  llvm::SmallVector<GenericTypeParamType *, 4> params(genericParams.begin(),
+                                                      genericParams.end());
+
+  struct AssociateMissingParams : public ASTWalker {
+    llvm::SmallVectorImpl<GenericTypeParamType *> &Params;
+    Callback Fn;
+
+    AssociateMissingParams(SmallVectorImpl<GenericTypeParamType *> &params,
+                           Callback callback)
+        : Params(params), Fn(callback) {}
+
+    bool walkToTypeReprPre(TypeRepr *T) override {
+      if (Params.empty())
+        return false;
+
+      auto *ident = dyn_cast<ComponentIdentTypeRepr>(T);
+      if (!ident)
+        return true;
+
+      auto *decl = dyn_cast_or_null<GenericTypeDecl>(ident->getBoundDecl());
+      if (!decl)
+        return true;
+
+      auto *paramList = decl->getGenericParams();
+      if (!paramList)
+        return true;
+
+      // There could a situation like `S<S>()`, so we need to be
+      // careful not to point at first `S` because it has all of
+      // its generic parameters specified.
+      if (auto *generic = dyn_cast<GenericIdentTypeRepr>(ident)) {
+        if (paramList->size() == generic->getNumGenericArgs())
+          return true;
+      }
+
+      for (auto *candidate : paramList->getParams()) {
+        auto result =
+            llvm::find_if(Params, [&](const GenericTypeParamType *param) {
+              return candidate == param->getDecl();
+            });
+
+        if (result != Params.end()) {
+          Fn(ident, *result);
+          Params.erase(result);
+        }
+      }
+
+      // Keep walking.
+      return true;
+    }
+  } paramAssociator(params, callback);
+
+  typeLoc.getTypeRepr()->walk(paramAssociator);
+  return params.empty();
+}

lib/Sema/CSFix.h

@@ -153,6 +153,11 @@ enum class FixKind : uint8_t {
   /// Remove `return` or default last expression of single expression
   /// function to `Void` to conform to expected result type.
   RemoveReturn,
+
+  /// Generic parameters could not be inferred and have to be explicitly
+  /// specified in the source. This fix groups all of the missing arguments
+  /// associated with single declaration.
+  ExplicitlySpecifyGenericArguments,
 };
 
 class ConstraintFix {
@@ -917,6 +922,53 @@ public:
          ConstraintLocator *locator);
 };
 
+class ExplicitlySpecifyGenericArguments final : public ConstraintFix {
+  using Anchor = llvm::PointerUnion<ConstraintLocator *, TypeRepr *>;
+
+  llvm::SmallDenseMap<Anchor, llvm::SmallVector<GenericTypeParamType *, 4>>
+      Parameters;
+
+  ExplicitlySpecifyGenericArguments(ConstraintSystem &cs,
+                                    ArrayRef<GenericTypeParamType *> params,
+                                    ConstraintLocator *locator)
+      : ConstraintFix(cs, FixKind::ExplicitlySpecifyGenericArguments, locator) {
+    assert(!params.empty());
+    Parameters[locator].append(params.begin(), params.end());
+  }
+
+  ExplicitlySpecifyGenericArguments(
+      ConstraintSystem &cs,
+      llvm::SmallDenseMap<TypeRepr *,
+                          llvm::SmallVector<GenericTypeParamType *, 4>> &params,
+      ConstraintLocator *locator)
+      : ConstraintFix(cs, FixKind::ExplicitlySpecifyGenericArguments, locator) {
+    assert(!params.empty());
+    for (const auto &elt : params) {
+      Parameters.insert(elt);
+    }
+  }
+
+public:
+  std::string getName() const override {
+    return "default missing generic argument to `Any`";
+  }
+
+  bool diagnose(Expr *root, bool asNote = false) const override;
+
+  static ExplicitlySpecifyGenericArguments *
+  create(ConstraintSystem &cs, ArrayRef<GenericTypeParamType *> params,
+         ConstraintLocator *locator);
+
+private:
+  /// Retrieve representative locations for given generic prameters
+  /// rooted at the given anchor.
+  ///
+  /// \returns true if all of the parameters have been covered.
+  static bool findArgumentLocations(
+      Expr *anchor, ArrayRef<GenericTypeParamType *> genericParams,
+      llvm::function_ref<void(TypeRepr *, GenericTypeParamType *)> callback);
+};
+
 } // end namespace constraints
 } // end namespace swift
 

lib/Sema/CSSimplify.cpp

@@ -6659,6 +6659,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
   case FixKind::AllowAnyObjectKeyPathRoot:
   case FixKind::TreatKeyPathSubscriptIndexAsHashable:
   case FixKind::AllowInvalidRefInKeyPath:
+  case FixKind::ExplicitlySpecifyGenericArguments:
     llvm_unreachable("handled elsewhere");
   }
 

lib/Sema/CSStep.cpp

@@ -275,8 +275,58 @@ StepResult ComponentStep::take(bool prevFailed) {
   // If there are no disjunctions or type variables to bind
   // we can't solve this system unless we have free type variables
   // allowed in the solution.
-  if (!CS.solverState->allowsFreeTypeVariables() && CS.hasFreeTypeVariables())
+  if (!CS.solverState->allowsFreeTypeVariables() && CS.hasFreeTypeVariables()) {
-    return done(/*isSuccess=*/false);
+    if (!CS.shouldAttemptFixes())
+      return done(/*isSuccess=*/false);
+
+    // Let's see if all of the free type variables are associated with
+    // generic parameters and if so, let's default them to `Any` and continue
+    // solving so we can properly diagnose the problem later by suggesting
+    // to explictly specify them.
+
+    llvm::SmallDenseMap<ConstraintLocator *,
+                        llvm::SmallVector<GenericTypeParamType *, 4>>
+        defaultableGenericParams;
+
+    for (auto *typeVar : CS.getTypeVariables()) {
+      if (typeVar->getImpl().hasRepresentativeOrFixed())
+        continue;
+
+      // If this free type variable is not a generic parameter
+      // we are done.
+      auto *locator = typeVar->getImpl().getLocator();
+
+      auto *anchor = locator->getAnchor();
+      if (!(anchor && locator->isForGenericParameter()))
+        return done(/*isSuccess=*/false);
+
+      // Increment the score for every missing generic argument
+      // to make ranking of the solutions with different number
+      // of generic arguments easier.
+      CS.increaseScore(ScoreKind::SK_Fix);
+      // Default argument to `Any`.
+      CS.assignFixedType(typeVar, CS.getASTContext().TheAnyType);
+      // Note that this generic argument has been given a default value.
+      CS.DefaultedConstraints.push_back(locator);
+
+      auto path = locator->getPath();
+      // Let's drop `generic parameter '...'` part of the locator to
+      // group all of the missing generic parameters related to the
+      // same path together.
+      defaultableGenericParams[CS.getConstraintLocator(anchor, path.drop_back(),
+                                                       /*summaryFlags=*/0)]
+          .push_back(locator->getGenericParameter());
+    }
+
+    for (const auto &missing : defaultableGenericParams) {
+      auto *locator = missing.first;
+      auto &missingParams = missing.second;
+      auto *fix =
+          ExplicitlySpecifyGenericArguments::create(CS, missingParams, locator);
+      if (CS.recordFix(fix))
+        return done(/*isSuccess=*/false);
+    }
+  }
 
   // If this solution is worse than the best solution we've seen so far,
   // skip it.

lib/Sema/ConstraintSystem.cpp

@@ -100,13 +100,9 @@ void ConstraintSystem::incrementLeafScopes() {
 
 bool ConstraintSystem::hasFreeTypeVariables() {
   // Look for any free type variables.
-  for (auto tv : TypeVariables) {
+  return llvm::any_of(TypeVariables, [](const TypeVariableType *typeVar) {
-    if (!tv->getImpl().hasRepresentativeOrFixed()) {
+    return !typeVar->getImpl().hasRepresentativeOrFixed();
-      return true;
+  });
-    }
-  }
-  
-  return false;
 }
 
 void ConstraintSystem::addTypeVariable(TypeVariableType *typeVar) {

test/Constraints/array_literal.swift

@@ -102,7 +102,7 @@ func longArray() {
   var _=["1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1"]
 }
 
-[1,2].map // expected-error {{expression type '((Int) throws -> _) throws -> [_]' is ambiguous without more context}}
+[1,2].map // expected-error {{generic parameter 'T' could not be inferred}}
 
 
 // <rdar://problem/25563498> Type checker crash assigning array literal to type conforming to ArrayProtocol

test/Constraints/bridging.swift

@@ -223,7 +223,9 @@ func takesDictionary<K, V>(_ p: Dictionary<K, V>) {} // expected-note {{in call
 func takesArray<T>(_ t: Array<T>) {} // expected-note {{in call to function 'takesArray'}}
 func rdar19695671() {
   takesSet(NSSet() as! Set) // expected-error{{generic parameter 'T' could not be inferred}}
-  takesDictionary(NSDictionary() as! Dictionary) // expected-error{{generic parameter 'K' could not be inferred}}
+  takesDictionary(NSDictionary() as! Dictionary)
+  // expected-error@-1 {{generic parameter 'K' could not be inferred}}
+  // expected-error@-2 {{generic parameter 'V' could not be inferred}}
   takesArray(NSArray() as! Array) // expected-error{{generic parameter 'T' could not be inferred}}
 }
 

test/Constraints/closures.swift

@@ -716,7 +716,7 @@ func rdar37790062() {
 }
 
 // <rdar://problem/39489003>
-typealias KeyedItem<K, T> = (key: K, value: T)
+typealias KeyedItem<K, T> = (key: K, value: T) // expected-note {{'T' declared as parameter to type 'KeyedItem'}}
 
 protocol Node {
   associatedtype T
@@ -730,8 +730,7 @@ extension Node {
   func getChild(for key:K)->(key: K, value: T) {
     return children.first(where: { (item:KeyedItem) -> Bool in
         return item.key == key
-        // expected-error@-1 {{binary operator '==' cannot be applied to operands of type '_' and 'Self.K'}}
+        // expected-error@-1 {{generic parameter 'T' could not be inferred}}
-        // expected-note@-2 {{overloads for '==' exist with these partially matching parameter lists:}}
       })!
   }
 }
@@ -814,13 +813,15 @@ func rdar_40537960() {
     init(_: P_40537960) {}
   }
 
-  struct A<T: Collection, P: P_40537960> {
+  struct A<T: Collection, P: P_40537960> { // expected-note {{'P' declared as parameter to type 'A'}}
     typealias Data = T.Element
     init(_: T, fn: (Data) -> R<P>) {}
   }
 
   var arr: [S] = []
-  _ = A(arr, fn: { L($0.v) }) // expected-error {{cannot convert value of type 'L' to closure result type 'R<T>'}}
+  _ = A(arr, fn: { L($0.v) }) // expected-error {{cannot convert value of type 'L' to closure result type 'R<Any>'}}
+  // expected-error@-1 {{generic parameter 'P' could not be inferred}}
+  // expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{8-8=<[S], <#P: P_40537960#>>}}
 }
 
 // rdar://problem/45659733

test/Constraints/diagnostics.swift

@@ -903,7 +903,7 @@ func test2208() {
 
 // SR-2164: Erroneous diagnostic when unable to infer generic type
 
-struct SR_2164<A, B> { // expected-note 3 {{'B' declared as parameter to type 'SR_2164'}} expected-note 2 {{'A' declared as parameter to type 'SR_2164'}} expected-note * {{generic type 'SR_2164' declared here}}
+struct SR_2164<A, B> { // expected-note 4 {{'B' declared as parameter to type 'SR_2164'}} expected-note 2 {{'A' declared as parameter to type 'SR_2164'}} expected-note * {{generic type 'SR_2164' declared here}}
   init(a: A) {}
   init(b: B) {}
   init(c: Int) {}
@@ -921,7 +921,10 @@ struct SR_2164_Dict<A: Hashable, B> { // expected-note {{'B' declared as paramet
 
 SR_2164(a: 0) // expected-error {{generic parameter 'B' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}}
 SR_2164(b: 1) // expected-error {{generic parameter 'A' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}}
-SR_2164(c: 2) // expected-error {{generic parameter 'A' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}}
+SR_2164(c: 2)
+// expected-error@-1 {{generic parameter 'A' could not be inferred}}
+// expected-error@-2 {{generic parameter 'B' could not be inferred}}
+// expected-note@-3 {{explicitly specify the generic arguments to fix this issue}} {{8-8=<Any, Any>}}
 SR_2164(3) // expected-error {{generic parameter 'B' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}}
 SR_2164_Array([4]) // expected-error {{generic parameter 'B' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}}
 SR_2164(e: 5) // expected-error {{generic parameter 'B' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}}

test/Constraints/generics.swift

@@ -52,7 +52,7 @@ func generic_metatypes<T : SomeProtocol>(_ x: T)
 }
 
 // Inferring a variable's type from a call to a generic.
-struct Pair<T, U> { } // expected-note 3 {{'T' declared as parameter to type 'Pair'}} expected-note 2 {{'U' declared as parameter to type 'Pair'}}
+struct Pair<T, U> { } // expected-note 3 {{'T' declared as parameter to type 'Pair'}} expected-note 3 {{'U' declared as parameter to type 'Pair'}}
 
 func pair<T, U>(_ x: T, _ y: U) -> Pair<T, U> { }
 
@@ -219,8 +219,7 @@ protocol P19215114 {}
 func body9215114<T: P19215114, U: Q19215114>(_ t: T) -> (_ u: U) -> () {}
 
 func test9215114<T: P19215114, U: Q19215114>(_ t: T) -> (U) -> () {
-  //Should complain about not being able to infer type of U.
+  let f = body9215114(t)  // expected-error {{generic parameter 'U' could not be inferred}}
-  let f = body9215114(t)  // expected-error {{generic parameter 'T' could not be inferred}}
   return f
 }
 
@@ -231,7 +230,7 @@ class Whatever<A: Numeric, B: Numeric> {  // expected-note 2 {{'A' declared as p
   static func bar() {}
 
 }
-Whatever.foo(a: 23) // expected-error {{generic parameter 'A' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}} {{9-9=<<#A: Numeric#>, <#B: Numeric#>>}}
+Whatever.foo(a: 23) // expected-error {{generic parameter 'A' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}} {{9-9=<<#A: Numeric#>, Int>}}
 
 // <rdar://problem/21718955> Swift useless error: cannot invoke 'foo' with no arguments
 Whatever.bar()  // expected-error {{generic parameter 'A' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}} {{9-9=<<#A: Numeric#>, <#B: Numeric#>>}}
@@ -262,7 +261,7 @@ protocol SubProto: BaseProto {}
   func copy() -> Any
 }
 
-struct FullyGeneric<Foo> {} // expected-note 11 {{'Foo' declared as parameter to type 'FullyGeneric'}} expected-note 1 {{generic type 'FullyGeneric' declared here}}
+struct FullyGeneric<Foo> {} // expected-note 13 {{'Foo' declared as parameter to type 'FullyGeneric'}} expected-note 1 {{generic type 'FullyGeneric' declared here}}
 
 struct AnyClassBound<Foo: AnyObject> {} // expected-note {{'Foo' declared as parameter to type 'AnyClassBound'}} expected-note {{generic type 'AnyClassBound' declared here}}
 // expected-note@-1{{requirement specified as 'Foo' : 'AnyObject'}}
@@ -340,7 +339,10 @@ func testFixIts() {
   // expected-error@-1 {{referencing initializer 'init()' on 'ClassAndProtosBound2' requires that 'X' conform to 'NSCopyish'}}
   // expected-error@-2 {{referencing initializer 'init()' on 'ClassAndProtosBound2' requires that 'X' conform to 'SubProto'}}
 
-  _ = Pair() // expected-error {{generic parameter 'T' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}} {{11-11=<Any, Any>}}
+  _ = Pair()
+  // expected-error@-1 {{generic parameter 'T' could not be inferred}}
+  // expected-error@-2 {{generic parameter 'U' could not be inferred}}
+  // expected-note@-3 {{explicitly specify the generic arguments to fix this issue}} {{11-11=<Any, Any>}}
   _ = Pair(first: S()) // expected-error {{generic parameter 'U' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}} {{11-11=<S, Any>}}
   _ = Pair(second: S()) // expected-error {{generic parameter 'T' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}} {{11-11=<Any, S>}}
 }
@@ -359,13 +361,13 @@ func testFixItClassBound() {
 }
 
 func testFixItCasting(x: Any) {
-  _ = x as! FullyGeneric // expected-error {{generic parameter 'Foo' could not be inferred in cast to 'FullyGeneric<_>'}} expected-note {{explicitly specify the generic arguments to fix this issue}} {{25-25=<Any>}}
+  _ = x as! FullyGeneric // expected-error {{generic parameter 'Foo' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}} {{25-25=<Any>}}
 }
 
 func testFixItContextualKnowledge() {
   // FIXME: These could propagate backwards.
-  let _: Int = Pair().first // expected-error {{generic parameter 'U' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}} {{20-20=<Any, Any>}}
+  let _: Int = Pair().first // expected-error {{generic parameter 'U' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}} {{20-20=<Int, Any>}}
-  let _: Int = Pair().second // expected-error {{generic parameter 'T' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}} {{20-20=<Any, Any>}}
+  let _: Int = Pair().second // expected-error {{generic parameter 'T' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}} {{20-20=<Any, Int>}}
 }
 
 func testFixItTypePosition() {
@@ -382,32 +384,47 @@ func testFixItNested() {
   _ = [FullyGeneric]() // expected-error {{generic parameter 'Foo' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}} {{20-20=<Any>}}
 
   _ = FullyGeneric<FullyGeneric>() // expected-error {{generic parameter 'Foo' could not be inferred}}
+  // expected-note@-1 {{explicitly specify the generic arguments to fix this issue}} {{32-32=<Any>}}
 
-  _ = Pair< // expected-error {{generic parameter 'Foo' could not be inferred}}
+  _ = Pair<
     FullyGeneric,
-    // expected-note@-1 {{explicitly specify the generic arguments to fix this issue}} {{17-17=<Any>}}
+    // expected-error@-1 {{generic parameter 'Foo' could not be inferred}}
-    FullyGeneric // FIXME: We could diagnose both of these, but we don't.
+    // expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{17-17=<Any>}}
+    FullyGeneric
+    // expected-error@-1 {{generic parameter 'Foo' could not be inferred}}
+    // expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{17-17=<Any>}}
   >()
-  _ = Pair< // expected-error {{generic parameter 'Foo' could not be inferred}}
+  _ = Pair<
     FullyGeneric<Any>,
     FullyGeneric
+    // expected-error@-1 {{generic parameter 'Foo' could not be inferred}}
+    // expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{17-17=<Any>}}
   >()
-  _ = Pair< // expected-error {{generic parameter 'Foo' could not be inferred}}
+  _ = Pair<
     FullyGeneric,
-    // expected-note@-1 {{explicitly specify the generic arguments to fix this issue}} {{17-17=<Any>}}
+    // expected-error@-1 {{generic parameter 'Foo' could not be inferred}}
+    // expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{17-17=<Any>}}
     FullyGeneric<Any>
   >()
 
-  _ = pair( // expected-error {{generic parameter 'Foo' could not be inferred}} {{none}}
+  _ = pair(
-    FullyGeneric(), // expected-note {{explicitly specify the generic arguments to fix this issue}}
+    FullyGeneric(),
+    // expected-error@-1 {{generic parameter 'Foo' could not be inferred}}
+    // expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{17-17=<Any>}}
     FullyGeneric()
+    // expected-error@-1 {{generic parameter 'Foo' could not be inferred}}
+    // expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{17-17=<Any>}}
   )
-  _ = pair( // expected-error {{generic parameter 'Foo' could not be inferred}} {{none}}
+  _ = pair(
     FullyGeneric<Any>(),
-    FullyGeneric() // expected-note {{explicitly specify the generic arguments to fix this issue}}
+    FullyGeneric()
+    // expected-error@-1 {{generic parameter 'Foo' could not be inferred}}
+    // expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{17-17=<Any>}}
   )
-  _ = pair( // expected-error {{generic parameter 'Foo' could not be inferred}} {{none}}
+  _ = pair(
-    FullyGeneric(), // expected-note {{explicitly specify the generic arguments to fix this issue}}
+    FullyGeneric(),
+    // expected-error@-1 {{generic parameter 'Foo' could not be inferred}}
+    // expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{17-17=<Any>}}
     FullyGeneric<Any>()
   )
 }
@@ -435,7 +452,7 @@ class GenericClass<A> {}
 func genericFunc<T>(t: T) {
   _ = [T: GenericClass] // expected-error {{generic parameter 'A' could not be inferred}}
   // expected-note@-1 {{explicitly specify the generic arguments to fix this issue}}
-  // expected-error@-2 3 {{type 'T' does not conform to protocol 'Hashable'}}
+  // expected-error@-2 {{type 'T' does not conform to protocol 'Hashable'}}
 }
 
 struct SR_3525<T> {}
@@ -707,3 +724,26 @@ struct SR10694 {
     // expected-error@-1 {{initializing from a metatype value must reference 'init' explicitly}}
   }
 }
+
+// SR-7003 (rdar://problem/51203824) - Poor diagnostics when attempting to access members on unfulfilled generic type
+func sr_7003() {
+  struct E<T> { // expected-note 4 {{'T' declared as parameter to type 'E'}}
+    static var foo: String { return "" }
+    var bar: String { return "" }
+    static func baz() -> String { return "" }
+    func qux() -> String { return "" }
+  }
+
+  let _: Any = E.foo
+  // expected-error@-1 {{generic parameter 'T' could not be inferred}}
+  // expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{17-17=<Any>}}
+  let _: Any = E().bar
+  // expected-error@-1 {{generic parameter 'T' could not be inferred}}
+  // expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{17-17=<Any>}}
+  let _: Any = E.baz()
+  // expected-error@-1 {{generic parameter 'T' could not be inferred}}
+  // expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{17-17=<Any>}}
+  let _: Any = E().qux()
+  // expected-error@-1 {{generic parameter 'T' could not be inferred}}
+  // expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{17-17=<Any>}}
+}

test/Constraints/incomplete_function_ref.swift

@@ -1,12 +1,13 @@
 // RUN: %target-typecheck-verify-swift
 
 struct MyCollection<Element> {  // expected-note {{'Element' declared as parameter to type 'MyCollection'}}
-  func map<T>(_ transform: (Element) -> T) -> MyCollection<T> {
+  func map<T>(_ transform: (Element) -> T) -> MyCollection<T> { // expected-note 2 {{in call to function 'map'}}
     fatalError("implement")
   }
 }
 
 MyCollection.map // expected-error{{generic parameter 'Element' could not be inferred}} expected-note {{explicitly specify the generic arguments to fix this issue}} {{13-13=<Any>}}
+// expected-error@-1 {{generic parameter 'T' could not be inferred}}
 
 let a = MyCollection<Int>()
 a.map // expected-error{{generic parameter 'T' could not be inferred}}

test/Constraints/keyword_arguments.swift

@@ -347,7 +347,9 @@ func trailingClosure5<T>(_ file: String = #file, line: UInt = #line, expression:
 func trailingClosure6<T>(value: Int, expression: () -> T?) { }
 
 trailingClosure5(file: "hello", line: 17) { return Optional.Some(5) } // expected-error{{extraneous argument label 'file:' in call}}{{18-24=}}
+// expected-error@-1 {{enum type 'Optional<Any>' has no case 'Some'; did you mean 'some'}} {{61-65=some}}
 trailingClosure6(5) { return Optional.Some(5) } // expected-error{{missing argument label 'value:' in call}}{{18-18=value: }}
+// expected-error@-1 {{enum type 'Optional<Any>' has no case 'Some'; did you mean 'some'}} {{39-43=some}}
 
 class MismatchOverloaded1 {
   func method1(_ x: Int!, arg: ((Int) -> Int)!) { }

test/Constraints/rdar46544601.swift

@@ -3,7 +3,7 @@
 struct D {}
 
 class Future<T> {
-  func then<U>(_ fn: @escaping (T) -> Future<U>) -> Future<U> { fatalError() }
+  func then<U>(_ fn: @escaping (T) -> Future<U>) -> Future<U> { fatalError() } // expected-note {{in call to function 'then'}}
   func thenThrowing<U>(_ fn: @escaping (T) throws -> U) -> Future<U> { fatalError() }
   func whenFailure(_ fn: @escaping (Error) -> Void) {}
 
@@ -12,21 +12,18 @@ class Future<T> {
 
 protocol P {
   func foo(arr: [D], data: ArraySlice<UInt8>) -> Future<D>
-  // expected-note@-1 {{found this candidate}}
   func bar(root: D, from: P) -> Future<D>
 }
 
 extension P {
   func foo(arr: [D] = [], data: [UInt8]) -> Future<D> { fatalError() }
-  // expected-note@-1 {{found this candidate}}
 }
 
 func crash(_ p: P, payload: [UInt8]) throws {
   p.foo(data: payload).then { _ in
     return Future<(D, [D])>()
-  }.then { (id, arr) in
+  }.then { (id, arr) in // expected-error {{generic parameter 'U' could not be inferred}}
     p.foo(arr: arr, data: []).and(result: (id, arr))
-    // expected-error@-1 {{mbiguous reference to member 'foo(arr:data:)'}}
   }.then { args0 in
     let (parentID, args1) = args0
     p.bar(root: parentID, from: p).and(args1)

test/Constraints/tuple.swift

@@ -219,13 +219,13 @@ extension r25271859 {
   func map<U>(f: (T) -> U) -> r25271859<U> {
   }
 
-  func andThen<U>(f: (T) -> r25271859<U>) {
+  func andThen<U>(f: (T) -> r25271859<U>) { // expected-note {{in call to function 'andThen(f:)'}}
   }
 }
 
 func f(a : r25271859<(Float, Int)>) {
-  a.map { $0.0 }
+  a.map { $0.0 } // expected-error {{generic parameter 'U' could not be inferred}} (This is related to how solver is setup with multiple statements)
-    .andThen { _ in   // expected-error {{unable to infer complex closure return type; add explicit type to disambiguate}} {{18-18=-> r25271859<String> }}
+    .andThen { _ in
       print("hello") // comment this out and it runs, leave any form of print in and it doesn't
       return r25271859<String>()
   }

test/Generics/invalid.swift

@@ -23,7 +23,7 @@ struct Lunch<T> {
   }
 }
 
-class Deli<Spices> {
+class Deli<Spices> { // expected-note {{'Spices' declared as parameter to type 'Deli'}}
 
   class Pepperoni {}
   struct Sausage {}
@@ -79,7 +79,8 @@ func badDiagnostic2() {
 func takesAny(_ a: Any) {}
 
 func badDiagnostic3() {
-  takesAny(Deli.self) // expected-error {{argument type 'Deli<_>.Type' does not conform to expected type 'Any'}}
+  takesAny(Deli.self) // expected-error {{generic parameter 'Spices' could not be inferred}}
+  // expected-note@-1 {{explicitly specify the generic arguments to fix this issue}} {{16-16=<Any>}}
 }
 
 // Crash with missing nested type inside concrete type

test/Sema/typo_correction.swift

@@ -121,7 +121,7 @@ protocol P {} // expected-error {{invalid redeclaration of 'P'}}
 // expected-note@-1 {{did you mean 'P'?}}
 
 func hasTypo() {
-  _ = P.a.a // expected-error {{type 'Generic<_>' has no member 'a'}}
+  _ = P.a.a // expected-error {{type 'Generic<Any>' has no member 'a'}}
 }
 
 // Typo correction with AnyObject.

test/decl/func/operator.swift

@@ -390,7 +390,8 @@ func testPostfixOperatorOnTuple<A, B>(a: A, b: B) {
   _ = (§)foo
   // expected-error@-1 {{consecutive statements on a line must be separated by ';'}}
   // expected-error@-2 {{generic parameter 'T' could not be inferred}}
-  // expected-warning@-3 {{expression of type '(A, (B, B), A)' is unused}}
+  // expected-error@-3 {{generic parameter 'U' could not be inferred}}
+  // expected-warning@-4 {{expression of type '(A, (B, B), A)' is unused}}
   _ = (§)(foo)
   _ = (a, (b, b), a)§
   _ = (§)(a, (b, b), a) // expected-error {{operator function '§' expects a single parameter of type '(T, (U, U), T)'}}

test/decl/nested/type_in_type.swift

@@ -377,7 +377,7 @@ protocol ExpressibleByDogLiteral {}
 struct Kitten : ExpressibleByCatLiteral {}
 struct Puppy : ExpressibleByDogLiteral {}
 
-struct Claws<A: ExpressibleByCatLiteral> {
+struct Claws<A: ExpressibleByCatLiteral> { // expected-note 3 {{'A' declared as parameter to type 'Claws'}}
   struct Fangs<B: ExpressibleByDogLiteral> { } // expected-note {{where 'B' = 'NotADog'}}
 }
 
@@ -387,7 +387,7 @@ func pets<T>(fur: T) -> Claws<Kitten>.Fangs<T> {
   return Claws<Kitten>.Fangs<T>()
 }
 
-func something<T>() -> T { // expected-note {{in call to function 'something()'}}
+func something<T>() -> T {
   while true {}
 }
 
@@ -398,11 +398,14 @@ func test() {
   let _: Claws.Fangs<Puppy> = pets(fur: Puppy())
   let _: Claws.Fangs<Puppy> = Claws<Kitten>.Fangs()
   let _: Claws.Fangs<Puppy> = Claws.Fangs()
-  // expected-error@-1 {{cannot convert value of type 'Claws<_>.Fangs<_>' to specified type 'Claws.Fangs<Puppy>'}}
+  // expected-error@-1 {{generic parameter 'A' could not be inferred}}
+  // expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{36-36=<<#A: ExpressibleByCatLiteral#>>}}
   let _: Claws.Fangs<NotADog> = something()
-  // expected-error@-1 {{generic parameter 'T' could not be inferred}} // FIXME: bad diagnostic
+  // expected-error@-1 {{generic parameter 'A' could not be inferred}}
-  _ = Claws.Fangs<NotADog>() // TODO(diagnostics): There should be two errors here - "cannot infer A" and "NotADog conformance to ExpressibleByDogLiteral"
+  _ = Claws.Fangs<NotADog>()
-  // expected-error@-1 {{referencing initializer 'init()' on 'Claws.Fangs' requires that 'NotADog' conform to 'ExpressibleByDogLiteral'}}
+  // expected-error@-1 {{generic parameter 'A' could not be inferred}}
+  // expected-error@-2 {{referencing initializer 'init()' on 'Claws.Fangs' requires that 'NotADog' conform to 'ExpressibleByDogLiteral'}}
+  // expected-note@-3 {{explicitly specify the generic arguments to fix this issue}} {{12-12=<<#A: ExpressibleByCatLiteral#>>}}
 }
 
 // https://bugs.swift.org/browse/SR-4379

test/decl/typealias/generic.swift

@@ -60,7 +60,7 @@ typealias B<T1> = MyType<T1, T1>  // expected-note {{'T1' declared as parameter
 typealias C<T> = MyType<String, T>
 
 // Type aliases with unused generic params.
-typealias D<T1, T2, T3> = MyType<T2, T1>  // expected-note {{'T3' declared as parameter to type 'D'}}
+typealias D<T1, T2, T3> = MyType<T2, T1>  // expected-note 3 {{'T3' declared as parameter to type 'D'}}
 
 typealias E<T1, T2> = Int  // expected-note {{generic type 'E' declared here}}
 
@@ -72,17 +72,18 @@ typealias G<S1, S2> = A<S1, S2>
 let _ : E<Int, Float> = 42
 let _ : E<Float> = 42   // expected-error {{generic type 'E' specialized with too few type parameters (got 1, but expected 2)}}
 let _ : E = 42   // expected-error {{cannot convert value of type 'Int' to specified type 'E'}}
-let _ : D = D(a: 1, b: 2)  // expected-error {{cannot convert value of type 'MyType<Int, Int>' to specified type 'D'}}
+let _ : D = D(a: 1, b: 2)
+// expected-error@-1 {{generic parameter 'T3' could not be inferred}}
+// expected-note@-2 {{explicitly specify the generic arguments to fix this issue}} {{14-14=<Int, Int, Any>}}
 
 let _ : D<Int, Int, Float> = D<Int, Int, Float>(a: 1, b: 2)
 
-// FIXME: This is not a great error.
-// expected-error @+1 {{cannot convert value of type 'D<Int, Int, Float>' (aka 'MyType<Int, Int>') to specified type 'D'}}
 let _ : D = D<Int, Int, Float>(a: 1, b: 2)
+// expected-error@-1 {{generic parameter 'T3' could not be inferred}}
 
 
 // expected-error @+2 {{generic parameter 'T3' could not be inferred}}
-// expected-note @+1 {{explicitly specify the generic arguments to fix this issue}} {{31-31=<Any, Any, Any>}}
+// expected-note @+1 {{explicitly specify the generic arguments to fix this issue}} {{31-31=<Int, Int, Any>}}
 let _ : D<Int, Int, Float> = D(a: 1, b: 2)
 
 let _ : F = { (a : Int) -> Int in a }  // Infer the types of F

test/expr/delayed-ident/enum.swift

@@ -15,7 +15,7 @@ e1 = .Third(5, 3.14159)
 e1 = .default  // SE-0071
 
 // Generic enumeration type
-enum E2<T> {
+enum E2<T> { // expected-note {{'T' declared as parameter to type 'E2'}}
   case First
   case Second(T)
 }

test/expr/unary/keypath/keypath.swift

@@ -36,7 +36,7 @@ struct A: Hashable {
   func hash(into hasher: inout Hasher) { fatalError() }
 }
 struct B {}
-struct C<T> {
+struct C<T> { // expected-note 2 {{'T' declared as parameter to type 'C'}}
   var value: T
   subscript() -> T { get { return value } }
   subscript(sub: Sub) -> T { get { return value } set { } }
@@ -190,7 +190,7 @@ func testKeyPath(sub: Sub, optSub: OptSub,
   let _: AnyKeyPath = \A.property
   let _: AnyKeyPath = \C<A>.value
   let _: AnyKeyPath = \.property // expected-error{{ambiguous}}
-  let _: AnyKeyPath = \C.value // expected-error{{cannot convert}} (need to improve diagnostic)
+  let _: AnyKeyPath = \C.value // expected-error{{generic parameter 'T' could not be inferred}}
   let _: AnyKeyPath = \.value // expected-error{{ambiguous}}
 
   let _ = \Prop.[nonHashableSub] // expected-error{{subscript index of type 'NonHashableSub' in a key path must be Hashable}}
@@ -282,9 +282,8 @@ func tupleGeneric<T, U>(_ v: (T, U)) {
 
   _ = ("tuple", "too", "big")[keyPath: tuple_el_1()]
   // expected-note@-12 {{}}
-  // expected-error@-2 {{cannot be applied}}
+  // expected-error@-2 {{generic parameter 'T' could not be inferred}}
-  // expected-error@-3 {{cannot be applied}}
+  // expected-error@-3 {{generic parameter 'U' could not be inferred}}
-  // expected-error@-4 {{could not be inferred}}
 }
 
 struct Z { }

test/stdlib/KeyPathAppending.swift

@@ -49,32 +49,32 @@ func mismatchedAppends<T, U, V>(readOnlyLeft: KeyPath<T, U>,
                                 readOnlyRight: KeyPath<U, V>,
                                 writableRight: WritableKeyPath<U, V>,
                                 referenceRight: ReferenceWritableKeyPath<U, V>){
-  // expected-error@+1{{}}
   _ = readOnlyRight.appending(path: readOnlyLeft)
+  // expected-error@-1 {{referencing instance method 'appending(path:)' on '_AppendKeyPath' requires the types 'KeyPath<U, V>' and 'AnyKeyPath' be equivalent}}
 
-  // expected-error@+1{{}}
   _ = readOnlyRight.appending(path: writableLeft)
+  // expected-error@-1 {{referencing instance method 'appending(path:)' on '_AppendKeyPath' requires the types 'KeyPath<U, V>' and 'AnyKeyPath' be equivalent}}
 
-  // expected-error@+1{{}}
   _ = readOnlyRight.appending(path: referenceLeft)
+  // expected-error@-1 {{referencing instance method 'appending(path:)' on '_AppendKeyPath' requires the types 'KeyPath<U, V>' and 'AnyKeyPath' be equivalent}}
 
-  // expected-error@+1{{}}
   _ = writableRight.appending(path: readOnlyLeft)
+  // expected-error@-1 {{referencing instance method 'appending(path:)' on '_AppendKeyPath' requires the types 'WritableKeyPath<U, V>' and 'AnyKeyPath' be equivalent}}
 
-  // expected-error@+1{{}}
   _ = writableRight.appending(path: writableLeft)
+  // expected-error@-1 {{referencing instance method 'appending(path:)' on '_AppendKeyPath' requires the types 'WritableKeyPath<U, V>' and 'AnyKeyPath' be equivalent}}
 
-  // expected-error@+1{{}}
   _ = writableRight.appending(path: referenceLeft)
+  // expected-error@-1 {{referencing instance method 'appending(path:)' on '_AppendKeyPath' requires the types 'WritableKeyPath<U, V>' and 'AnyKeyPath' be equivalent}}
 
-  // expected-error@+1{{}}
   _ = referenceRight.appending(path: readOnlyLeft)
+  // expected-error@-1 {{referencing instance method 'appending(path:)' on '_AppendKeyPath' requires the types 'ReferenceWritableKeyPath<U, V>' and 'AnyKeyPath' be equivalent}}
 
-  // expected-error@+1{{}}
   _ = referenceRight.appending(path: writableLeft)
+  // expected-error@-1 {{referencing instance method 'appending(path:)' on '_AppendKeyPath' requires the types 'ReferenceWritableKeyPath<U, V>' and 'AnyKeyPath' be equivalent}}
 
-  // expected-error@+1{{}}
   _ = referenceRight.appending(path: referenceLeft)
+  // expected-error@-1 {{referencing instance method 'appending(path:)' on '_AppendKeyPath' requires the types 'ReferenceWritableKeyPath<U, V>' and 'AnyKeyPath' be equivalent}}
 }
 
 func partialAppends<T, U, V>(partial: PartialKeyPath<T>,