No longer hang on typeof of a function type

We were calling `isFunctionProtoType()` on a `ParsedType` rather than
creating a valid semantic type first and calling the function on that.
The call to `isFunctionProtoType()` would eventually call
`getUnqualifiedDesugaredType()`, which loops indefinitely until we get
a desugared type and a `ParsedType` will never finish desugaring.

Fixes #64713

clang/docs/ReleaseNotes.rst

@@ -144,7 +144,10 @@ Bug Fixes in This Version
   module may end up with members associated with the wrong declaration of the
   class, which can result in miscompiles in some cases.
 - Fix crash on use of a variadic overloaded operator.
-  (`#42535 <https://github.com/llvm/llvm-project/issues/42535>`_)
+  (`#42535 <https://github.com/llvm/llvm-project/issues/42535>_`)
+- Fix a hang on valid C code passing a function type as an argument to
+  ``typeof`` to form a function declaration.
+  (`#64713 <https://github.com/llvm/llvm-project/issues/64713>_`)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDecl.cpp

@@ -9235,7 +9235,8 @@ static FunctionDecl *CreateNewFunctionDecl(Sema &SemaRef, Declarator &D,
     bool HasPrototype =
         (D.isFunctionDeclarator() && D.getFunctionTypeInfo().hasPrototype) ||
         (D.getDeclSpec().isTypeRep() &&
-         D.getDeclSpec().getRepAsType().get()->isFunctionProtoType()) ||
+         SemaRef.GetTypeFromParser(D.getDeclSpec().getRepAsType(), nullptr)
+             ->isFunctionProtoType()) ||
         (!R->getAsAdjusted<FunctionType>() && R->isFunctionProtoType());
     assert(
         (HasPrototype || !SemaRef.getLangOpts().requiresStrictPrototypes()) &&

clang/test/C/C2x/n2927.c

@@ -90,3 +90,7 @@ extern typeof(D) C;                // C has type "double[2]"
 typeof(D) D = { 5, 8.9, 0.1, 99 }; // D is now completed to "double[4]"
 extern double E[4];
 extern typeof(D) E;                // E has type "double[4]" from D’s completed type
+
+// GH64713 -- this used to trigger an infinite loop when creating the function
+// declaration for F from the function designator specified by typeof.
+typeof(int(int)) F;                // F has type "int(int)"