Revert "Sema: Remove dead diagnostic"

This reverts commit d56d704.

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -7563,6 +7563,13 @@ ERROR(result_builder_buildpartialblock_accumulated_not_accessible,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

@@ -2852,6 +2852,22 @@ bool ContextualFailure::diagnoseAsError() {
     break;
   }
 
+  case ConstraintLocator::OptionalInjection: {
+    // 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->isCGFloat()) &&
+          (toType->isDouble() || toType->isCGFloat())) {
+        fromType = OptionalType::get(fromType);
+        toType = OptionalType::get(toType);
+        diagnostic = diag::cannot_implicitly_convert_in_optional_context;
+        break;
+      }
+    }
+
+    return false;
+  }
+
   case ConstraintLocator::EnumPatternImplicitCastMatch: {
     // In this case, the types are reversed, as we are checking whether we
     // can convert the pattern type to the context type.