Index: clang/lib/AST/Mangle.cpp
===================================================================
--- clang/lib/AST/Mangle.cpp
+++ clang/lib/AST/Mangle.cpp
@@ -225,11 +225,17 @@
   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);
 }
 
Index: clang/test/CodeGen/pr52782-stdcall-func-decl.cpp
===================================================================
--- /dev/null
+++ clang/test/CodeGen/pr52782-stdcall-func-decl.cpp
@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -triple i686-w64-windows-gnu -o - -emit-llvm -debug-info-kind=constructor %s
+
+#define NS_IMETHOD_(type) type __stdcall
+
+enum nsresult {};
+
+class NotNull;
+
+class nsICanvasRenderingContextInternal {
+  NS_IMETHOD_(nsresult) InitializeWithDrawTarget(NotNull);
+} nsTBaseHashSet;