[clang][CGDebugInfo] Don't generate an implicit 'this' parameter if one was specified explicitly (#100767)

Currently we would unconditionally add an implicit `this` parameter when
creating an instance method type. However, when we have an explicit
'this', we shouldn't generate one. This patch only passes a valid
`ThisPtr` type to `getOrCreateInstanceMethodType` if one wasn't
explicitly specified. There's no way to get a pointer to a member
function with an explicit `this` parameter (those are treated as regular
function pointers instead). So there's no need to account for that case
in `CGDebugInfo::CreateType`.

Fixes #99744

Author: Michael137

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -1942,7 +1942,12 @@ CGDebugInfo::getOrCreateMethodType(const CXXMethodDecl *Method,
   if (Method->isStatic())
     return cast_or_null<llvm::DISubroutineType>(
         getOrCreateType(QualType(Func, 0), Unit));
-  return getOrCreateInstanceMethodType(Method->getThisType(), Func, Unit);
+
+  QualType ThisType;
+  if (!Method->hasCXXExplicitFunctionObjectParameter())
+    ThisType = Method->getThisType();
+
+  return getOrCreateInstanceMethodType(ThisType, Func, Unit);
 }
 
 llvm::DISubroutineType *CGDebugInfo::getOrCreateInstanceMethodType(
@@ -1974,27 +1979,31 @@ llvm::DISubroutineType *CGDebugInfo::getOrCreateInstanceMethodType(
   Elts.push_back(Args[0]);
 
   // "this" pointer is always first argument.
-  const CXXRecordDecl *RD = ThisPtr->getPointeeCXXRecordDecl();
-  if (isa<ClassTemplateSpecializationDecl>(RD)) {
-    // Create pointer type directly in this case.
-    const PointerType *ThisPtrTy = cast<PointerType>(ThisPtr);
-    uint64_t Size = CGM.getContext().getTypeSize(ThisPtrTy);
-    auto Align = getTypeAlignIfRequired(ThisPtrTy, CGM.getContext());
-    llvm::DIType *PointeeType =
-        getOrCreateType(ThisPtrTy->getPointeeType(), Unit);
-    llvm::DIType *ThisPtrType =
-        DBuilder.createPointerType(PointeeType, Size, Align);
-    TypeCache[ThisPtr.getAsOpaquePtr()].reset(ThisPtrType);
-    // TODO: This and the artificial type below are misleading, the
-    // types aren't artificial the argument is, but the current
-    // metadata doesn't represent that.
-    ThisPtrType = DBuilder.createObjectPointerType(ThisPtrType);
-    Elts.push_back(ThisPtrType);
-  } else {
-    llvm::DIType *ThisPtrType = getOrCreateType(ThisPtr, Unit);
-    TypeCache[ThisPtr.getAsOpaquePtr()].reset(ThisPtrType);
-    ThisPtrType = DBuilder.createObjectPointerType(ThisPtrType);
-    Elts.push_back(ThisPtrType);
+  // ThisPtr may be null if the member function has an explicit 'this'
+  // parameter.
+  if (!ThisPtr.isNull()) {
+    const CXXRecordDecl *RD = ThisPtr->getPointeeCXXRecordDecl();
+    if (isa<ClassTemplateSpecializationDecl>(RD)) {
+      // Create pointer type directly in this case.
+      const PointerType *ThisPtrTy = cast<PointerType>(ThisPtr);
+      uint64_t Size = CGM.getContext().getTypeSize(ThisPtrTy);
+      auto Align = getTypeAlignIfRequired(ThisPtrTy, CGM.getContext());
+      llvm::DIType *PointeeType =
+          getOrCreateType(ThisPtrTy->getPointeeType(), Unit);
+      llvm::DIType *ThisPtrType =
+          DBuilder.createPointerType(PointeeType, Size, Align);
+      TypeCache[ThisPtr.getAsOpaquePtr()].reset(ThisPtrType);
+      // TODO: This and the artificial type below are misleading, the
+      // types aren't artificial the argument is, but the current
+      // metadata doesn't represent that.
+      ThisPtrType = DBuilder.createObjectPointerType(ThisPtrType);
+      Elts.push_back(ThisPtrType);
+    } else {
+      llvm::DIType *ThisPtrType = getOrCreateType(ThisPtr, Unit);
+      TypeCache[ThisPtr.getAsOpaquePtr()].reset(ThisPtrType);
+      ThisPtrType = DBuilder.createObjectPointerType(ThisPtrType);
+      Elts.push_back(ThisPtrType);
+    }
   }
 
   // Copy rest of the arguments.

clang/test/CodeGenCXX/debug-info-explicit-this.cpp

@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -emit-llvm -debug-info-kind=limited -std=c++2b %s -o - | FileCheck %s
+
+struct Foo {
+  void Bar(this Foo&& self) {}
+};
+
+void fn() {
+  Foo{}.Bar();
+}
+
+// CHECK: distinct !DISubprogram(name: "Bar", {{.*}}, type: ![[BAR_TYPE:[0-9]+]], {{.*}}, declaration: ![[BAR_DECL:[0-9]+]], {{.*}}
+// CHECK: ![[FOO:[0-9]+]] = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "Foo"
+// CHECK: ![[BAR_DECL]] = !DISubprogram(name: "Bar", {{.*}}, type: ![[BAR_TYPE]], {{.*}},
+// CHECK: ![[BAR_TYPE]] = !DISubroutineType(types: ![[PARAMS:[0-9]+]])
+// CHECK: ![[PARAMS]] = !{null, ![[SELF:[0-9]+]]}
+// CHECK: ![[SELF]] = !DIDerivedType(tag: DW_TAG_rvalue_reference_type, baseType: ![[FOO]]