[CS] Don't simplify FunctionResult in simplifyLocator

Simplifying into the function expression is wrong,
like `FunctionArgument` this isn't an element that
can be simplified. This also means we don't need
to handle it in `MissingCallFailure`, since we
shouldn't be recording that fix in this case.

Author: hamishknight

lib/Sema/CSDiagnostics.cpp

@@ -3903,14 +3903,6 @@ bool MissingCallFailure::diagnoseAsError() {
       return true;
     }
 
-    case ConstraintLocator::FunctionResult: {
-      path = path.drop_back();
-      if (path.back().getKind() != ConstraintLocator::AutoclosureResult)
-        break;
-
-      LLVM_FALLTHROUGH;
-    }
-
     case ConstraintLocator::AutoclosureResult: {
       auto loc = getConstraintLocator(getRawAnchor(), path.drop_back());
       AutoClosureForwardingFailure failure(getSolution(), loc);

lib/Sema/ConstraintSystem.cpp

@@ -3389,8 +3389,7 @@ void constraints::simplifyLocator(ASTNode &anchor,
       continue;
     }
 
-    case ConstraintLocator::ApplyFunction:
-    case ConstraintLocator::FunctionResult:
+    case ConstraintLocator::ApplyFunction: {
       // Extract application function.
       if (auto applyExpr = getAsExpr<ApplyExpr>(anchor)) {
         anchor = applyExpr->getFn();
@@ -3412,7 +3411,7 @@ void constraints::simplifyLocator(ASTNode &anchor,
       }
 
       break;
-
+    }
     case ConstraintLocator::AutoclosureResult:
     case ConstraintLocator::LValueConversion:
     case ConstraintLocator::DynamicType:
@@ -3677,6 +3676,7 @@ void constraints::simplifyLocator(ASTNode &anchor,
     case ConstraintLocator::SynthesizedArgument:
       break;
 
+    case ConstraintLocator::FunctionResult:
     case ConstraintLocator::DynamicLookupResult:
     case ConstraintLocator::KeyPathComponentResult:
       break;

test/Constraints/issue-78376.swift

@@ -0,0 +1,15 @@
+// RUN: %target-typecheck-verify-swift %clang-importer-sdk
+
+// REQUIRES: objc_interop
+
+import Foundation
+import CoreGraphics
+
+func foo(_ x: Double) {}
+func bar() -> Double { 0 }
+
+// https://github.com/swiftlang/swift/issues/78376
+let _: (CGFloat) -> Void = foo
+// expected-error@-1 {{cannot convert value of type '(Double) -> ()' to specified type '(CGFloat) -> Void'}}
+let _: () -> CGFloat = bar
+// expected-error@-1 {{cannot convert value of type '() -> Double' to specified type '() -> CGFloat'}}