Fix PR#62594 : static lambda call operator is not convertible to function pointer on win32

See issue #62594

This code does not work on win32:

  auto lstatic = []()  static  { return 0;  };
  int (*f2)(void) = lstatic;

Since a calling convention such as CC_X86ThisCall can rightly interfere with the implicit pointer to function conversion if erroneously marked on a static function, the fix entails checking the 'static' specifier on the lambda declarator prior to assigning it a calling convention of an non-static member (which pre-c++23 made sense).

Author: faisalv

clang/docs/ReleaseNotes.rst

@@ -283,6 +283,10 @@ Bug Fixes to C++ Support
 - Clang now properly handles out of line template specializations when there is
   a non-template inner-class between the function and the class template.
   (`#65810 <https://github.com/llvm/llvm-project/issues/65810>`_)
+  
+- Clang now properly converts static lambda call operator to function 
+  pointer on win32.
+  (`#62594 <https://github.com/llvm/llvm-project/issues/62594>`_)  
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaType.cpp

@@ -4095,8 +4095,9 @@ static CallingConv getCCForDeclaratorChunk(
           D.getTypeObject(I).Kind == DeclaratorChunk::MemberPointer;
     } else if (D.getContext() == DeclaratorContext::LambdaExpr) {
       // This can only be a call operator for a lambda, which is an instance
-      // method.
-      IsCXXInstanceMethod = true;
+      // method, unless explicitly specified as 'static'.
+      IsCXXInstanceMethod =
+          D.getDeclSpec().getStorageClassSpec() != DeclSpec::SCS_static;
     } else {
       // We're the innermost decl chunk, so must be a function declarator.
       assert(D.isFunctionDeclarator());

clang/test/SemaCXX/cxx23-static-callop-lambda-expression.cpp

@@ -0,0 +1,33 @@
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify %s
+
+
+namespace ns1 {
+  auto lstatic = []() static { return 3; }; 
+  int (*f2)(void) = lstatic;   
+    
+}
+
+namespace ns1_1 {
+  
+  auto lstatic = []() static consteval  //expected-error{{cannot take address of consteval call}} \
+                                          expected-note {{declared here}} 
+  { return 3; };   
+  
+  // FIXME: the above error should indicate that it was triggered below.
+  int (*f2)(void) = lstatic;   
+    
+}
+
+
+namespace ns2 {
+  auto lstatic = []() static { return 3; }; 
+  constexpr int (*f2)(void) = lstatic;                              
+  static_assert(lstatic() == f2());
+}
+
+namespace ns3 {
+  void main() {
+    static int x = 10;
+    auto L = []() static { return x; };
+  }
+}