[Sema] Convert placeholder types in expression conversion types

Author: Jumhyn

lib/Sema/CSGen.cpp

@@ -3802,16 +3802,28 @@ bool ConstraintSystem::generateConstraints(
       // Determine whether we know more about the contextual type.
       ContextualTypePurpose ctp = target.getExprContextualTypePurpose();
       bool isOpaqueReturnType = target.infersOpaqueReturnType();
+      auto *convertTypeLocator =
+          getConstraintLocator(expr, LocatorPathElt::ContextualType());
 
-      // Substitute type variables in for unresolved types.
+      // Substitute type variables in for placeholder types (and unresolved
+      // types, if allowed).
       if (allowFreeTypeVariables == FreeTypeVariableBinding::UnresolvedType) {
-        auto *convertTypeLocator =
-            getConstraintLocator(expr, LocatorPathElt::ContextualType());
-
         convertType = convertType.transform([&](Type type) -> Type {
-          if (type->is<UnresolvedType>()) {
-            return createTypeVariable(
-                convertTypeLocator, TVO_CanBindToNoEscape);
+          if (type->is<UnresolvedType>() || type->is<PlaceholderType>()) {
+            return createTypeVariable(convertTypeLocator,
+                                      TVO_CanBindToNoEscape |
+                                          TVO_PrefersSubtypeBinding |
+                                          TVO_CanBindToHole);
+          }
+          return type;
+        });
+      } else {
+        convertType = convertType.transform([&](Type type) -> Type {
+          if (type->is<PlaceholderType>()) {
+            return createTypeVariable(convertTypeLocator,
+                                      TVO_CanBindToNoEscape |
+                                          TVO_PrefersSubtypeBinding |
+                                          TVO_CanBindToHole);
           }
           return type;
         });