Enter the function parameter mangling scope for a function encoding

before mangling the template argument list.

With an abbreviated function template, the template parameters can
contain constraints that refer to the function parameters, so we need to
bring the function parameters into scope earlier.

Fixes llvm#67356.

Author: zygoloid

clang/lib/AST/ItaniumMangle.cpp

@@ -841,8 +841,17 @@ void CXXNameMangler::mangleFunctionEncoding(GlobalDecl GD) {
 
   AbiTagList ReturnTypeAbiTags = makeFunctionReturnTypeTags(FD);
   if (ReturnTypeAbiTags.empty()) {
-    // There are no tags for return type, the simplest case.
+    // There are no tags for return type, the simplest case. Enter the function
+    // parameter scope before mangling the name, because a template using
+    // constrained `auto` can have references to its parameters within its
+    // template argument list:
+    //
+    //   template<typename T> void f(T x, C<decltype(x)> auto)
+    // ... is mangled as ...
+    //   template<typename T, C<decltype(param 1)> U> void f(T, U)
+    FunctionTypeDepthState Saved = FunctionTypeDepth.push();
     mangleName(GD);
+    FunctionTypeDepth.pop(Saved);
     mangleFunctionEncodingBareType(FD);
     return;
   }
@@ -855,7 +864,10 @@ void CXXNameMangler::mangleFunctionEncoding(GlobalDecl GD) {
   CXXNameMangler FunctionEncodingMangler(*this, FunctionEncodingStream);
   // Output name of the function.
   FunctionEncodingMangler.disableDerivedAbiTags();
+
+  FunctionTypeDepthState Saved = FunctionTypeDepth.push();
   FunctionEncodingMangler.mangleNameWithAbiTags(FD, nullptr);
+  FunctionTypeDepth.pop(Saved);
 
   // Remember length of the function name in the buffer.
   size_t EncodingPositionStart = FunctionEncodingStream.str().size();
@@ -873,7 +885,9 @@ void CXXNameMangler::mangleFunctionEncoding(GlobalDecl GD) {
       AdditionalAbiTags.end());
 
   // Output name with implicit tags and function encoding from temporary buffer.
+  Saved = FunctionTypeDepth.push();
   mangleNameWithAbiTags(FD, &AdditionalAbiTags);
+  FunctionTypeDepth.pop(Saved);
   Out << FunctionEncodingStream.str().substr(EncodingPositionStart);
 
   // Function encoding could create new substitutions so we have to add

clang/test/CodeGenCXX/mangle-concept.cpp

@@ -228,3 +228,15 @@ namespace gh67244 {
   // CHECK: define {{.*}} @_ZN7gh672441fITkNS_1CIifEEiEEvT_(
   template void f(int);
 }
+
+namespace gh67356 {
+  template<typename, typename T> concept C = true;
+  template<typename T> void f(T t, C<decltype(t)> auto) {}
+  // CHECK: define {{.*}} @_ZN7gh673561fIiTkNS_1CIDtfL0p_EEEiEEvT_T0_(
+  template void f(int, int);
+
+  // Note, we use `fL0p` not `fp` above because:
+  template<typename T> void g(T t, C<auto (T u) -> decltype(f(t, u))> auto) {}
+  // CHECK: define {{.*}} @_ZN7gh673561gIiTkNS_1CIFDTcl1ffL0p_fp_EET_EEEiEEvS3_T0_(
+  template void g(int, int);
+}