[Diagnostics] A tailored diagnostic for Double<->CGFloat conversion via optional chaining

Implicit conversion between optional types are not currently supported,
this includes optional chaining:

```
struct S {
  var test: CGFloat
}

func compute(s: Double?) -> Double? {
  return 42
}

func test(s: S?) {
  _ = compute(s?.test) // CGFloat? -> Double? implicit conversion
}
```

Since `S` is optional, `test` ends up optional as well and that
wasn't correctly detected, so solver was allowed to form a correct
solution because `CGFloat` is converted to `Double` inside of
the chain _prior_ to it being wrapped into an optional because
contextual type of `Double` is propagated to the result of the
chain earlier that conversion takes place.

Resolves: rdar://83666783

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -5950,6 +5950,13 @@ ERROR(cannot_declare_computed_var_in_result_builder,none,
         "expression shuffles the elements of this tuple; "
         "this behavior is deprecated", ())
 
+//------------------------------------------------------------------------------
+// MARK: Implicit conversion diagnostics
+//------------------------------------------------------------------------------
+ERROR(cannot_implicitly_convert_in_optional_context,none,
+      "cannot implicitly convert value of type %0 to expected type %1",
+      (Type, Type))
+
 //------------------------------------------------------------------------------
 // MARK: marker protocol diagnostics
 //------------------------------------------------------------------------------

lib/Sema/CSDiagnostics.cpp

@@ -2418,6 +2418,22 @@ bool ContextualFailure::diagnoseAsError() {
     break;
   }
 
+  case ConstraintLocator::OptionalPayload: {
+    // If this is an attempt at a Double <-> CGFloat conversion
+    // through optional chaining, let's produce a tailored diagnostic.
+    if (isExpr<OptionalEvaluationExpr>(getAnchor())) {
+      if ((fromType->isDouble() || fromType->isCGFloatType()) &&
+          (toType->isDouble() || toType->isCGFloatType())) {
+        fromType = OptionalType::get(fromType);
+        toType = OptionalType::get(toType);
+        diagnostic = diag::cannot_implicitly_convert_in_optional_context;
+        break;
+      }
+    }
+
+    return false;
+  }
+
   default:
     return false;
   }