[CS] Improve diagnostics for non-metatype type(of:) contextual type

Emit a custom diagnostic for this case, and handle holes.

include/swift/AST/DiagnosticsSema.def

@@ -359,6 +359,11 @@ ERROR(cannot_convert_return_type_to_anyobject,none,
 ERROR(cannot_convert_to_return_type_nil,none,
       "'nil' is incompatible with return type %0", (Type))
 
+ERROR(cannot_convert_metatype_to_non_metatype,none,
+      "cannot convert metatype %0 to non-metatype %1", (Type, Type))
+ERROR(cannot_convert_typeof_to_non_metatype,none,
+      "cannot convert 'type(of:)' metatype to non-metatype %0", (Type))
+
 ERROR(cannot_convert_thrown_type,none,
       "thrown expression type %0 %select{cannot be converted to error type %1|"
       "does not conform to 'Error'}2",

include/swift/Sema/CSFix.h

@@ -394,6 +394,9 @@ enum class FixKind : uint8_t {
   /// Ignore a type imposed by an assignment destination e.g. `let x: Int = ...`
   IgnoreAssignmentDestinationType,
 
+  /// Ignore a non-metatype contextual type for a `type(of:)` expression.
+  IgnoreNonMetatypeDynamicType,
+
   /// Allow argument-to-parameter subtyping even when parameter type
   /// is marked as `inout`.
   AllowConversionThroughInOut,
@@ -2434,6 +2437,30 @@ public:
   }
 };
 
+/// Ignore a non-metatype contextual type for a `type(of:)` expression, for
+/// example `let x: Int = type(of: foo)`.
+class IgnoreNonMetatypeDynamicType final : public ContextualMismatch {
+  IgnoreNonMetatypeDynamicType(ConstraintSystem &cs, Type instanceTy,
+                               Type metatypeTy, ConstraintLocator *locator)
+      : ContextualMismatch(cs, FixKind::IgnoreNonMetatypeDynamicType,
+                           instanceTy, metatypeTy, locator) {}
+
+public:
+  std::string getName() const override {
+    return "ignore non-metatype result for 'type(of:)'";
+  }
+
+  bool diagnose(const Solution &solution, bool asNote = false) const override;
+
+  static IgnoreNonMetatypeDynamicType *create(ConstraintSystem &cs,
+                                              Type instanceTy, Type metatypeTy,
+                                              ConstraintLocator *locator);
+
+  static bool classof(const ConstraintFix *fix) {
+    return fix->getKind() == FixKind::IgnoreNonMetatypeDynamicType;
+  }
+};
+
 /// If this is an argument-to-parameter conversion which is associated with
 /// `inout` parameter, subtyping is not permitted, types have to
 /// be identical.

lib/Sema/CSDiagnostics.cpp

@@ -8189,6 +8189,20 @@ bool AssignmentTypeMismatchFailure::diagnoseAsNote() {
   return false;
 }
 
+bool NonMetatypeDynamicTypeFailure::diagnoseAsError() {
+  auto instanceTy = getFromType();
+  auto metatypeTy = getToType();
+  if (instanceTy->isBareErrorType()) {
+    emitDiagnostic(diag::cannot_convert_typeof_to_non_metatype, metatypeTy)
+        .highlight(getSourceRange());
+  } else {
+    emitDiagnostic(diag::cannot_convert_metatype_to_non_metatype,
+                   MetatypeType::get(instanceTy), metatypeTy)
+        .highlight(getSourceRange());
+  }
+  return true;
+}
+
 bool MissingContextualBaseInMemberRefFailure::diagnoseAsError() {
   auto *anchor = castToExpr(getAnchor());
   // Member reference could be wrapped into a number of parens

lib/Sema/CSDiagnostics.h

@@ -2428,6 +2428,15 @@ private:
   bool diagnoseMissingConformance() const;
 };
 
+class NonMetatypeDynamicTypeFailure final : public ContextualFailure {
+public:
+  NonMetatypeDynamicTypeFailure(const Solution &solution, Type instanceTy,
+                                Type metatypeTy, ConstraintLocator *locator)
+      : ContextualFailure(solution, instanceTy, metatypeTy, locator) {}
+
+  bool diagnoseAsError() override;
+};
+
 class MissingContextualBaseInMemberRefFailure final : public FailureDiagnostic {
   DeclNameRef MemberName;
 

lib/Sema/CSFix.cpp

@@ -1825,6 +1825,21 @@ IgnoreAssignmentDestinationType::create(ConstraintSystem &cs, Type sourceTy,
       IgnoreAssignmentDestinationType(cs, sourceTy, destTy, locator);
 }
 
+IgnoreNonMetatypeDynamicType *
+IgnoreNonMetatypeDynamicType::create(ConstraintSystem &cs, Type instanceTy,
+                                     Type metatypeTy,
+                                     ConstraintLocator *locator) {
+  return new (cs.getAllocator())
+      IgnoreNonMetatypeDynamicType(cs, instanceTy, metatypeTy, locator);
+}
+
+bool IgnoreNonMetatypeDynamicType::diagnose(const Solution &solution,
+                                            bool asNote) const {
+  NonMetatypeDynamicTypeFailure failure(solution, getFromType(), getToType(),
+                                        getLocator());
+  return failure.diagnose(asNote);
+}
+
 bool AllowInOutConversion::diagnose(const Solution &solution,
                                     bool asNote) const {
   InOutConversionFailure failure(solution, getFromType(), getToType(),

lib/Sema/CSSimplify.cpp

@@ -12541,8 +12541,24 @@ ConstraintSystem::simplifyDynamicTypeOfConstraint(
                       locator);
   }
 
-  // It's definitely not either kind of metatype, so we can
-  // report failure right away.
+  // We don't have a non-metatype result, produce a fix.
+  if (shouldAttemptFixes()) {
+    // If we have a hole as a contextual type, eagerly produce holes in the
+    // argument of `type(of:)`.
+    if (type1->isPlaceholder()) {
+      recordTypeVariablesAsHoles(type2);
+      return SolutionKind::Solved;
+    }
+
+    // Otherwise we have some invalid contextual type, record a fix and let the
+    // argument be turned into a hole if needed.
+    recordAnyTypeVarAsPotentialHole(type2);
+
+    recordFix(IgnoreNonMetatypeDynamicType::create(
+        *this, type2, type1, getConstraintLocator(locator)));
+    return SolutionKind::Solved;
+  }
+
   return SolutionKind::Error;
 }
 
@@ -16100,6 +16116,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
   case FixKind::IgnoreMissingEachKeyword:
   case FixKind::AllowInlineArrayLiteralCountMismatch:
   case FixKind::TooManyDynamicMemberLookups:
+  case FixKind::IgnoreNonMetatypeDynamicType:
   case FixKind::IgnoreIsolatedConformance:
     llvm_unreachable("handled elsewhere");
   }

test/Constraints/type_of.swift

@@ -87,3 +87,30 @@ do {
     }
   }
 }
+
+_ = { x in // expected-error {{cannot infer type of closure parameter 'x' without a type annotation}}
+  let _: Undefined = Swift.type(of: x)
+  // expected-error@-1 {{cannot find type 'Undefined' in scope}}
+}
+_ = {
+  func foo<T>() -> T {}
+  let _: Undefined = Swift.type(of: foo())
+  // expected-error@-1 {{cannot find type 'Undefined' in scope}}
+}
+_ = {
+  let _: Undefined = Swift.type(of: .foo)
+  // expected-error@-1 {{cannot find type 'Undefined' in scope}}
+}
+let _: Int = Swift.type(of: .foo)
+// expected-error@-1 {{cannot convert 'type(of:)' metatype to non-metatype 'Int'}}
+
+let _ = Swift.type(of: .foo)
+// expected-error@-1 {{cannot infer contextual base in reference to member 'foo'}}
+
+// FIXME: Ideally we'd include the type of the argument in the diagnostic, currently
+// we bind it to a hole before we open the closure.
+let _: Int = Swift.type(of: { (); return 0 }())
+// expected-error@-1 {{cannot convert 'type(of:)' metatype to non-metatype 'Int'}}
+
+let _: Undefined = Swift.type(of: { (); return 0 }())
+// expected-error@-1 {{cannot find type 'Undefined' in scope}}