[clang][#52782] Bail on incomplete parameter type in stdcall name mangling

stdcall name mangling requires a suffix with the number equal to the sum of the byte count of all parameter types. In the case of a function prototype that has a parameter type of an incomplete type it is impossible to get the size of the type. While such a function is not callable or able to be defined in the TU, it may still be mangled when generating debug info, which would previously lead to a crash.
This patch fixes that by simply bailing out of the loop and using the so far accumulated byte count. This matches GCCs behaviour as well: bc8d6c6013/gcc/config/i386/winnt.c (L203)

Fixes https://github.com/llvm/llvm-project/issues/52782

Differential Revision: https://reviews.llvm.org/D116020

clang/lib/AST/Mangle.cpp

@@ -225,11 +225,17 @@ void MangleContext::mangleName(GlobalDecl GD, raw_ostream &Out) {
   if (const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(FD))
     if (!MD->isStatic())
       ++ArgWords;
-  for (const auto &AT : Proto->param_types())
+  for (const auto &AT : Proto->param_types()) {
+    // If an argument type is incomplete there is no way to get its size to
+    // correctly encode into the mangling scheme.
+    // Follow GCCs behaviour by simply breaking out of the loop.
+    if (AT->isIncompleteType())
+      break;
     // Size should be aligned to pointer size.
     ArgWords +=
         llvm::alignTo(ASTContext.getTypeSize(AT), TI.getPointerWidth(0)) /
         TI.getPointerWidth(0);
+  }
   Out << ((TI.getPointerWidth(0) / 8) * ArgWords);
 }
 

clang/test/CodeGen/pr52782-stdcall-func-decl.cpp

@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -triple i686-w64-windows-gnu -o - -emit-llvm -debug-info-kind=constructor %s | FileCheck %s
+
+enum nsresult {};
+
+class NotNull;
+
+class nsICanvasRenderingContextInternal {
+  // CHECK: !DISubprogram(name: "InitializeWithDrawTarget", linkageName: "\01__ZN33nsICanvasRenderingContextInternal24InitializeWithDrawTargetE7NotNull@4"
+  nsresult __stdcall InitializeWithDrawTarget(NotNull);
+} nsTBaseHashSet;