[ConstraintSystem] Record generic fix if destination of a pointer conversion is invalid

If the right-hand side (destination) of value-to-pointer conversion
is incorrect e.g. base type of member is a hole, let's record
a generic "invalid conversion" failure.

Resolves: rdar://problem/68254165

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -3829,8 +3829,11 @@ bool ConstraintSystem::repairFailures(
     // If this is an implicit 'something-to-pointer' conversion
     // it's going to be diagnosed by specialized fix which deals
     // with generic argument mismatches.
-    if (matchKind == ConstraintKind::BindToPointerType)
-      break;
+    if (matchKind == ConstraintKind::BindToPointerType) {
+      auto *member = rhs->getAs<DependentMemberType>();
+      if (!(member && member->getBase()->hasHole()))
+        break;
+    }
 
     // If this is a ~= operator implicitly generated by pattern matching
     // let's not try to fix right-hand side of the operator because it's

test/Constraints/valid_pointer_conversions.swift

@@ -41,3 +41,9 @@ func SR12382(_ x: UnsafeMutablePointer<Double>??) {}
 var i = 0
 SR12382(&i) // expected-error {{cannot convert value of type 'UnsafeMutablePointer<Int>' to expected argument type 'UnsafeMutablePointer<Double>'}}
 // expected-note@-1 {{arguments to generic parameter 'Pointee' ('Int' and 'Double') are expected to be equal}}
+
+//problem/68254165 - Bad diagnostic when using String init(decodingCString:) with an incorrect pointer type
+func rdar68254165(ptr: UnsafeMutablePointer<Int8>) {
+  _ = String(decodingCString: ptr, as: .utf8) // expected-error {{generic parameter 'Encoding' could not be inferred}}
+  // expected-error@-1 {{type '_.Type' has no member 'utf8'}}
+}