🍒 [5.7] Sema, DiagQoI: Fix and tailor diagnosis of explicit closure result type conflicts

include/swift/AST/DiagnosticsSema.def

@@ -288,9 +288,12 @@ FIXIT(insert_closure_return_type_placeholder,
 NOTE(use_of_anon_closure_param,none,
      "anonymous closure parameter %0 is used here", (Identifier))
 
-ERROR(incorrect_explicit_closure_result,none,
+ERROR(incorrect_explicit_closure_result_vs_contextual_type,none,
       "declared closure result %0 is incompatible with contextual type %1",
       (Type, Type))
+ERROR(incorrect_explicit_closure_result_vs_return_type,none,
+      "declared closure result %0 is incompatible with return type %1",
+      (Type, Type))
 
 ERROR(unsupported_closure_attr,none,
       "%select{attribute |}0 '%1' is not supported on a closure",

include/swift/Sema/ConstraintLocatorPathElts.def

@@ -54,6 +54,9 @@ SIMPLE_LOCATOR_PATH_ELT(AutoclosureResult)
 /// The result of a closure.
 SIMPLE_LOCATOR_PATH_ELT(ClosureResult)
 
+/// FIXME: Misleading name: this locator is used only for single-expression
+/// closure returns.
+///
 /// The type of a closure body.
 CUSTOM_LOCATOR_PATH_ELT(ClosureBody)
 

lib/Sema/CSDiagnostics.cpp

@@ -2318,17 +2318,32 @@ bool ContextualFailure::diagnoseAsError() {
   auto toType = getToType();
 
   Diag<Type, Type> diagnostic;
-  switch (path.back().getKind()) {
+
+  const auto lastPathEltKind = path.back().getKind();
+  switch (lastPathEltKind) {
   case ConstraintLocator::ClosureBody:
   case ConstraintLocator::ClosureResult: {
     auto *closure = castToExpr<ClosureExpr>(getRawAnchor());
     if (closure->hasExplicitResultType() &&
         closure->getExplicitResultTypeRepr()) {
       auto resultRepr = closure->getExplicitResultTypeRepr();
-      emitDiagnosticAt(resultRepr->getStartLoc(),
-                       diag::incorrect_explicit_closure_result, fromType,
-                       toType)
-          .fixItReplace(resultRepr->getSourceRange(), toType.getString());
+
+      if (lastPathEltKind == ConstraintLocator::ClosureBody) {
+        // The conflict is between the return type and the declared result type.
+        emitDiagnosticAt(resultRepr->getStartLoc(),
+                         diag::incorrect_explicit_closure_result_vs_return_type,
+                         toType, fromType)
+            .fixItReplace(resultRepr->getSourceRange(), fromType.getString());
+      } else {
+        // The conflict is between the declared result type and the
+        // contextual type.
+        emitDiagnosticAt(
+            resultRepr->getStartLoc(),
+            diag::incorrect_explicit_closure_result_vs_contextual_type,
+            fromType, toType)
+            .fixItReplace(resultRepr->getSourceRange(), toType.getString());
+      }
+
       return true;
     }
 

test/Constraints/closures.swift

@@ -996,7 +996,7 @@ func rdar52204414() {
   let _: () -> Void = { return 42 }
   // expected-error@-1 {{cannot convert value of type 'Int' to closure result type 'Void'}}
   let _ = { () -> Void in return 42 }
-  // expected-error@-1 {{declared closure result 'Int' is incompatible with contextual type 'Void'}}
+  // expected-error@-1 {{declared closure result 'Void' is incompatible with return type 'Int'}} {{19-23=Int}}
 }
 
 // SR-12291 - trailing closure is used as an argument to the last (positionally) parameter.

test/Sema/substring_to_string_conversion_swift4.swift

@@ -45,7 +45,7 @@ do {
 
 // CTP_ClosureResult
 do {
-  [ss].map { (x: Substring) -> String in x } // expected-error {{declared closure result 'Substring' is incompatible with contextual type 'String'}}
+  [ss].map { (x: Substring) -> String in x } // expected-error {{declared closure result 'String' is incompatible with return type 'Substring'}} {{32-38=Substring}}
 }
 
 // CTP_ArrayElement