[Diagnostic] Don't fix partial mismatch for sub-types associated with…

… optional conversion

If mismatch detected by `repairFailures` is related to a complex
wrapped value of optional type formed from optional-to-optional
or value-to-optional conversion let's not try to fix it directly
but let `simplifyRestrictedConstraintImpl` record a top-level fix
for more context.

Resolves: rdar://problem/59703585

lib/Sema/CSDiagnostics.cpp

@@ -1884,6 +1884,12 @@ bool ContextualFailure::diagnoseAsError() {
       return true;
     }
 
+    if (isa<AssignExpr>(anchor)) {
+      emitDiagnostic(anchor->getLoc(), diag::cannot_convert_assign,
+                     getFromType(), getToType());
+      return true;
+    }
+
     return false;
   }
 

lib/Sema/CSSimplify.cpp

@@ -3788,6 +3788,12 @@ bool ConstraintSystem::repairFailures(
         tupleLocator->isLastElement<LocatorPathElt::GenericArgument>())
       break;
 
+    // If the mismatch is a part of either optional-to-optional or
+    // value-to-optional conversions, let's allow fix refer to a complete
+    // top level type and not just a part of it.
+    if (tupleLocator->findLast<LocatorPathElt::OptionalPayload>())
+      break;
+
     ConstraintFix *fix;
     if (tupleLocator->isLastElement<LocatorPathElt::FunctionArgument>()) {
       fix = AllowFunctionTypeMismatch::create(*this, lhs, rhs, tupleLocator, index);

test/Constraints/function_conversion.swift

@@ -68,3 +68,15 @@ noEscapeParam = takesAny // expected-error {{converting non-escaping value to 'A
 func rdar_59773317(x: () -> Int) -> (() -> Int)? { // expected-note {{parameter 'x' is implicitly non-escaping}}
   return x // expected-error {{using non-escaping parameter 'x' in a context expecting an @escaping closure}}
 }
+
+// rdar://problem/59703585 - Wrong error message when signature of a C function type and Swift implementation mismatch
+func rdar_59703585() {
+  typealias Fn = @convention(c) (UnsafePointer<Int8>?, UnsafeMutableRawPointer?) -> Void
+
+  func swiftCallback(someString: UnsafePointer<Int8>, someObject: UnsafeMutableRawPointer?) {}
+
+  var cb: Fn? = nil
+
+  cb = swiftCallback
+  // expected-error@-1 {{cannot assign value of type '(UnsafePointer<Int8>, UnsafeMutableRawPointer?) -> ()' to type 'Fn?' (aka 'Optional<@convention(c) (Optional<UnsafePointer<Int8>>, Optional<UnsafeMutableRawPointer>) -> ()>')}}
+}