[ConstraintSystem] Repair and diagnose failures relared to throws mismatch

If the only difference between two functions is `throws` and it
is not a subtype relationship, let's repair the problem by dropping
`throws` attribute and letting solver continue to search for
a solution, which would later be diagnosed.

lib/Sema/CSDiagnostics.cpp

@@ -1858,19 +1858,6 @@ bool ContextualFailure::diagnoseAsError() {
     if (diagnoseConversionToBool())
       return true;
 
-    auto *fnType1 = getFromType()->getAs<FunctionType>();
-    auto *fnType2 = getToType()->getAs<FunctionType>();
-
-    if (fnType1 && fnType2) {
-      if (fnType1->throws() != fnType2->throws()) {
-        auto throwingTy = fnType1->throws() ? fnType1 : fnType2;
-        emitDiagnostic(anchor->getLoc(), diag::throws_functiontype_mismatch,
-                       throwingTy, throwingTy == fnType1 ? fnType2 : fnType1)
-            .highlight(anchor->getSourceRange());
-        return true;
-      }
-    }
-
     if (auto msg = getDiagnosticFor(getContextualTypePurpose(),
                                     /*forProtocol=*/false)) {
       diagnostic = *msg;
@@ -4084,3 +4071,10 @@ bool InvalidTupleSplatWithSingleParameterFailure::diagnoseAsError() {
       .fixItInsert(argExpr->getEndLoc(), ")");
   return true;
 }
+
+bool ThrowingFunctionConversionFailure::diagnoseAsError() {
+  auto *anchor = getAnchor();
+  emitDiagnostic(anchor->getLoc(), diag::throws_functiontype_mismatch,
+                 getFromType(), getToType());
+  return true;
+}

lib/Sema/CSDiagnostics.h

@@ -764,6 +764,28 @@ protected:
   getDiagnosticFor(ContextualTypePurpose context, bool forProtocol);
 };
 
+/// Diagnose failures related to conversion between throwing function type
+/// and non-throwing one e.g.
+///
+/// ```swift
+/// func foo<T>(_ t: T) throws -> Void {}
+/// let _: (Int) -> Void = foo // `foo` can't be implictly converted to
+///                            // non-throwing type `(Int) -> Void`
+/// ```
+class ThrowingFunctionConversionFailure final : public ContextualFailure {
+public:
+  ThrowingFunctionConversionFailure(Expr *root, ConstraintSystem &cs,
+                                    Type fromType, Type toType,
+                                    ConstraintLocator *locator)
+      : ContextualFailure(root, cs, fromType, toType, locator) {
+    auto fnType1 = fromType->castTo<FunctionType>();
+    auto fnType2 = toType->castTo<FunctionType>();
+    assert(fnType1->throws() != fnType2->throws());
+  }
+
+  bool diagnoseAsError() override;
+};
+
 /// Diagnose failures related attempt to implicitly convert types which
 /// do not support such implicit converstion.
 /// "as" or "as!" has to be specified explicitly in cases like that.

lib/Sema/CSFix.cpp

@@ -744,6 +744,21 @@ bool AllowTupleSplatForSingleParameter::attempt(
   return cs.recordFix(fix);
 }
 
+bool DropThrowsAttribute::diagnose(Expr *root, bool asNote) const {
+  auto &cs = getConstraintSystem();
+  ThrowingFunctionConversionFailure failure(root, cs, getFromType(),
+                                            getToType(), getLocator());
+  return failure.diagnose(asNote);
+}
+
+DropThrowsAttribute *DropThrowsAttribute::create(ConstraintSystem &cs,
+                                                 FunctionType *fromType,
+                                                 FunctionType *toType,
+                                                 ConstraintLocator *locator) {
+  return new (cs.getAllocator())
+      DropThrowsAttribute(cs, fromType, toType, locator);
+}
+
 bool IgnoreContextualType::diagnose(Expr *root, bool asNote) const {
   auto &cs = getConstraintSystem();
   ContextualFailure failure(root, cs, getFromType(), getToType(), getLocator());

lib/Sema/CSFix.h

@@ -494,6 +494,26 @@ public:
                                     ConstraintLocator *locator);
 };
 
+/// This is a contextual mismatch between throwing and non-throwing
+/// function types, repair it by dropping `throws` attribute.
+class DropThrowsAttribute final : public ContextualMismatch {
+  DropThrowsAttribute(ConstraintSystem &cs, FunctionType *fromType,
+                      FunctionType *toType, ConstraintLocator *locator)
+      : ContextualMismatch(cs, fromType, toType, locator) {
+    assert(fromType->throws() != toType->throws());
+  }
+
+public:
+  std::string getName() const override { return "drop 'throws' attribute"; }
+
+  bool diagnose(Expr *root, bool asNote = false) const override;
+
+  static DropThrowsAttribute *create(ConstraintSystem &cs,
+                                     FunctionType *fromType,
+                                     FunctionType *toType,
+                                     ConstraintLocator *locator);
+};
+
 /// Append 'as! T' to force a downcast to the specified type.
 class ForceDowncast final : public ContextualMismatch {
   ForceDowncast(ConstraintSystem &cs, Type fromType, Type toType,

lib/Sema/CSSimplify.cpp

@@ -1323,8 +1323,15 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
   // A non-throwing function can be a subtype of a throwing function.
   if (func1->throws() != func2->throws()) {
     // Cannot drop 'throws'.
-    if (func1->throws() || kind < ConstraintKind::Subtype)
+    if (func1->throws() || kind < ConstraintKind::Subtype) {
-      return getTypeMatchFailure(locator);
+      if (!shouldAttemptFixes())
+        return getTypeMatchFailure(locator);
+
+      auto *fix = DropThrowsAttribute::create(*this, func1, func2,
+                                              getConstraintLocator(locator));
+      if (recordFix(fix))
+        return getTypeMatchFailure(locator);
+    }
   }
 
   // A non-@noescape function type can be a subtype of a @noescape function
@@ -7154,8 +7161,8 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
     // been diagnosed as "missing explicit call", let's
     // increase the score to make sure that we don't impede that.
     if (auto *fnType = type1->getAs<FunctionType>()) {
-      auto result =
+      auto result = matchTypes(fnType->getResult(), type2, matchKind,
-          matchTypes(fnType->getResult(), type2, matchKind, subflags, locator);
+                               TMF_ApplyingFix, locator);
       if (result == SolutionKind::Solved)
         increaseScore(SK_Fix);
     }

test/Constraints/function.swift

@@ -53,7 +53,7 @@ func process(_ line: UInt = #line) -> Int { return 0 }
 func dangerous() throws {}
 
 func test() {
-  process {         // expected-error {{invalid conversion from throwing function of type '() throws -> ()' to non-throwing function type '() -> Void'}}
+  process {         // expected-error {{invalid conversion from throwing function of type '() throws -> Void' to non-throwing function type '() -> Void'}}
     try dangerous()
     test()
   }

test/Constraints/tuple_arguments.swift

@@ -1411,7 +1411,7 @@ func processArrayOfFunctions(f1: [((Bool, Bool)) -> ()],
   }
 
   f2.forEach { (block: ((Bool, Bool)) -> ()) in
-  // expected-error@-1 {{cannot convert value of type '(((Bool, Bool)) -> ()) -> ()' to expected argument type '(@escaping (Bool, Bool) -> ()) -> Void}}
+  // expected-error@-1 {{cannot convert value of type '(((Bool, Bool)) -> ()) -> ()' to expected argument type '(@escaping (Bool, Bool) -> ()) throws -> Void'}}
     block(p)
     block((c, c))
     block(c, c)

test/decl/func/throwing_functions.swift

@@ -51,7 +51,7 @@ func partialApply2<T: Parallelogram>(_ t: T) {
 func barG<T>(_ t : T) throws -> T { return t }
 func fooG<T>(_ t : T) -> T { return t }
 
-var bGE: (_ i: Int) -> Int = barG // expected-error{{invalid conversion from throwing function of type '(_) throws -> _' to non-throwing function type '(Int) -> Int'}}
+var bGE: (_ i: Int) -> Int = barG // expected-error{{invalid conversion from throwing function of type '(Int) throws -> Int' to non-throwing function type '(Int) -> Int'}}
 var bg: (_ i: Int) throws -> Int = barG
 var fG: (_ i: Int) throws -> Int = fooG
 
@@ -69,15 +69,15 @@ func fooT(_ callback: () throws -> Bool) {} //OK
 func fooT(_ callback: () -> Bool) {}
 
 // Throwing and non-throwing types are not equivalent.
-struct X<T> { } // expected-note {{arguments to generic parameter 'T' ('(String) -> Int' and '(String) throws -> Int') are expected to be equal}}
+struct X<T> { }
-// expected-note@-1 {{arguments to generic parameter 'T' ('(String) throws -> Int' and '(String) -> Int') are expected to be equal}}
+
 func specializedOnFuncType1(_ x: X<(String) throws -> Int>) { }
 func specializedOnFuncType2(_ x: X<(String) -> Int>) { }
 func testSpecializedOnFuncType(_ xThrows: X<(String) throws -> Int>,
                                xNonThrows: X<(String) -> Int>) {
   specializedOnFuncType1(xThrows) // ok
-  specializedOnFuncType1(xNonThrows) // expected-error{{cannot convert value of type 'X<(String) -> Int>' to expected argument type 'X<(String) throws -> Int>'}}
+  specializedOnFuncType1(xNonThrows) // expected-error{{invalid conversion from throwing function of type '(String) -> Int' to non-throwing function type '(String) throws -> Int'}}
-  specializedOnFuncType2(xThrows)  // expected-error{{cannot convert value of type 'X<(String) throws -> Int>' to expected argument type 'X<(String) -> Int>'}}
+  specializedOnFuncType2(xThrows)  // expected-error{{invalid conversion from throwing function of type '(String) throws -> Int' to non-throwing function type '(String) -> Int'}}
   specializedOnFuncType2(xNonThrows) // ok
 }
 
@@ -85,7 +85,7 @@ func testSpecializedOnFuncType(_ xThrows: X<(String) throws -> Int>,
 func subtypeResult1(_ x: (String) -> ((Int) -> String)) { }
 func testSubtypeResult1(_ x1: (String) -> ((Int) throws -> String),
                         x2: (String) -> ((Int) -> String)) {
-  subtypeResult1(x1) // expected-error{{cannot convert value of type '(String) -> ((Int) throws -> String)' to expected argument type '(String) -> ((Int) -> String)'}}
+  subtypeResult1(x1) // expected-error{{invalid conversion from throwing function of type '(Int) throws -> String' to non-throwing function type '(Int) -> String'}}
   subtypeResult1(x2)
 }
 
@@ -106,7 +106,7 @@ func testSubtypeArgument1(_ x1: (_ fn: ((String) -> Int)) -> Int,
 func subtypeArgument2(_ x: (_ fn: ((String) throws -> Int)) -> Int) { }
 func testSubtypeArgument2(_ x1: (_ fn: ((String) -> Int)) -> Int,
                           x2: (_ fn: ((String) throws -> Int)) -> Int) {
-  subtypeArgument2(x1) // expected-error{{cannot convert value of type '(((String) -> Int)) -> Int' to expected argument type '(((String) throws -> Int)) -> Int'}}
+  subtypeArgument2(x1) // expected-error{{invalid conversion from throwing function of type '(String) throws -> Int' to non-throwing function type '(String) -> Int'}}
   subtypeArgument2(x2)
 }
 
@@ -117,10 +117,10 @@ var c3 : () -> Int = c1 // expected-error{{invalid conversion from throwing func
 var c4 : () -> Int = {() throws -> Int in 0} // expected-error{{invalid conversion from throwing function of type '() throws -> Int' to non-throwing function type '() -> Int'}}
 var c5 : () -> Int = { try c2() } // expected-error{{invalid conversion from throwing function of type '() throws -> Int' to non-throwing function type '() -> Int'}}
 var c6 : () throws -> Int = { do { _ = try c2() } ; return 0 }
-var c7 : () -> Int = { do { try c2() } ; return 0 } // expected-error{{invalid conversion from throwing function of type '() throws -> _' to non-throwing function type '() -> Int'}}
+var c7 : () -> Int = { do { try c2() } ; return 0 } // expected-error{{invalid conversion from throwing function of type '() throws -> Int' to non-throwing function type '() -> Int'}}
 var c8 : () -> Int = { do { _ = try c2()  } catch _ { var x = 0 } ; return 0 } // expected-warning {{initialization of variable 'x' was never used; consider replacing with assignment to '_' or removing it}}
-var c9 : () -> Int = { do { try c2()  } catch Exception.A { var x = 0 } ; return 0 }// expected-error{{invalid conversion from throwing function of type '() throws -> _' to non-throwing function type '() -> Int'}}
+var c9 : () -> Int = { do { try c2()  } catch Exception.A { var x = 0 } ; return 0 }// expected-error{{invalid conversion from throwing function of type '() throws -> Int' to non-throwing function type '() -> Int'}}
-var c10 : () -> Int = { throw Exception.A; return 0 } // expected-error{{invalid conversion from throwing function of type '() throws -> _' to non-throwing function type '() -> Int'}}
+var c10 : () -> Int = { throw Exception.A; return 0 } // expected-error{{invalid conversion from throwing function of type '() throws -> Int' to non-throwing function type '() -> Int'}}
 var c11 : () -> Int = { try! c2() }
 var c12 : () -> Int? = { try? c2() }
 

test/stmt/errors.swift

@@ -145,7 +145,7 @@ func eleven_two() {
 enum Twelve { case Payload(Int) }
 func twelve_helper(_ fn: (Int, Int) -> ()) {}
 func twelve() {
-  twelve_helper { (a, b) in // expected-error {{invalid conversion from throwing function of type '(_, _) throws -> ()' to non-throwing function type '(Int, Int) -> ()'}}
+  twelve_helper { (a, b) in // expected-error {{invalid conversion from throwing function of type '(Int, Int) throws -> ()' to non-throwing function type '(Int, Int) -> ()'}}
     do {
       try thrower()
     } catch Twelve.Payload(a...b) {
@@ -158,7 +158,7 @@ func ==(a: Thirteen, b: Thirteen) -> Bool { return true }
 
 func thirteen_helper(_ fn: (Thirteen) -> ()) {}
 func thirteen() {
-  thirteen_helper { (a) in // expected-error {{invalid conversion from throwing function of type '(_) throws -> ()' to non-throwing function type '(Thirteen) -> ()'}}
+  thirteen_helper { (a) in // expected-error {{invalid conversion from throwing function of type '(Thirteen) throws -> ()' to non-throwing function type '(Thirteen) -> ()'}}
     do {
       try thrower()
     } catch a {