[clang-cl][AST] Fix auto NTTP MSVC 1920+ mangling for pointer types (#92477)

https://godbolt.org/z/G1K8Wszn9 for reference.

Starting with MSVC 1920+, VS2019, C++17 auto NTTP now adds `M <type>` to
the mangled name to avoid name collisions with different deduced types.
This PR fixes pointers. Pointers to members will be fixed in an upcoming
PR.

Here is a small example. The godbolt has more thorough examples.
```
template<auto>
struct AutoParmTemplate
{
    AutoParmTemplate() {}
};

int i;

int main()
{
    // MSVC 1916: ??0?$AutoParmTemplate@$1?i@@3HA@@QEAA@XZ
    // MSVC 1929: ??0?$AutoParmTemplate@$MPEAH1?i@@3HA@@QEAA@XZ
    // Clang: ??0?$AutoParmTemplate@$1?i@@3HA@@QEAA@XZ
    AutoParmTemplate<&i> x;
}
```

3 files changed, 98 insertions, 3 deletions

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 0973db2..88969a0 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -95,6 +95,13 @@ ABI Changes in This Version
 - Fixed Microsoft calling convention when returning classes that have a deleted
   copy assignment operator. Such a class should be returned indirectly.
 
+- Fixed Microsoft name mangling for auto non-type template arguments of pointer
+  type for MSVC 1920+. This change resolves incompatibilities with code compiled
+  by MSVC 1920+ but will introduce incompatibilities with code compiled by
+  earlier versions of Clang unless such code is built with the compiler option
+  `-fms-compatibility-version=19.14` to imitate the MSVC 1914 mangling behavior.
+
+
 AST Dumping Potentially Breaking Changes
 ----------------------------------------
 
diff --git a/clang/lib/AST/MicrosoftMangle.cpp b/clang/lib/AST/MicrosoftMangle.cpp
index d87be5f..3923d34 100644
--- a/clang/lib/AST/MicrosoftMangle.cpp
+++ b/clang/lib/AST/MicrosoftMangle.cpp
@@ -372,6 +372,11 @@ public:
   void mangleMemberFunctionPointer(const CXXRecordDecl *RD,
                                    const CXXMethodDecl *MD,
                                    StringRef Prefix = "$");
+  void mangleFunctionPointer(const FunctionDecl *FD,
+                             const NonTypeTemplateParmDecl *PD,
+                             QualType TemplateArgType);
+  void mangleVarDecl(const VarDecl *VD, const NonTypeTemplateParmDecl *PD,
+                     QualType TemplateArgType);
   void mangleMemberFunctionPointerInClassNTTP(const CXXRecordDecl *RD,
                                               const CXXMethodDecl *MD);
   void mangleVirtualMemPtrThunk(const CXXMethodDecl *MD,
@@ -799,6 +804,50 @@ MicrosoftCXXNameMangler::mangleMemberFunctionPointer(const CXXRecordDecl *RD,
     mangleNumber(VBTableOffset);
 }
 
+void MicrosoftCXXNameMangler::mangleFunctionPointer(
+    const FunctionDecl *FD, const NonTypeTemplateParmDecl *PD,
+    QualType TemplateArgType) {
+  // <func-ptr> ::= $1? <mangled-name>
+  // <func-ptr> ::= <auto-nttp>
+  //
+  // <auto-nttp> ::= $ M <type> 1? <mangled-name>
+  Out << '$';
+
+  if (getASTContext().getLangOpts().isCompatibleWithMSVC(
+          LangOptions::MSVC2019) &&
+      PD && PD->getType()->getTypeClass() == Type::Auto &&
+      !TemplateArgType.isNull()) {
+    Out << "M";
+    mangleType(TemplateArgType, SourceRange(), QMM_Drop);
+  }
+
+  Out << "1?";
+  mangleName(FD);
+  mangleFunctionEncoding(FD, /*ShouldMangle=*/true);
+}
+
+void MicrosoftCXXNameMangler::mangleVarDecl(const VarDecl *VD,
+                                            const NonTypeTemplateParmDecl *PD,
+                                            QualType TemplateArgType) {
+  // <var-ptr> ::= $1? <mangled-name>
+  // <var-ptr> ::= <auto-nttp>
+  //
+  // <auto-nttp> ::= $ M <type> 1? <mangled-name>
+  Out << '$';
+
+  if (getASTContext().getLangOpts().isCompatibleWithMSVC(
+          LangOptions::MSVC2019) &&
+      PD && PD->getType()->getTypeClass() == Type::Auto &&
+      !TemplateArgType.isNull()) {
+    Out << "M";
+    mangleType(TemplateArgType, SourceRange(), QMM_Drop);
+  }
+
+  Out << "1?";
+  mangleName(VD);
+  mangleVariableEncoding(VD);
+}
+
 void MicrosoftCXXNameMangler::mangleMemberFunctionPointerInClassNTTP(
     const CXXRecordDecl *RD, const CXXMethodDecl *MD) {
   // <nttp-class-member-function-pointer> ::= <member-function-pointer>
@@ -1555,6 +1604,9 @@ void MicrosoftCXXNameMangler::mangleIntegerLiteral(
     const llvm::APSInt &Value, const NonTypeTemplateParmDecl *PD,
     QualType TemplateArgType) {
   // <integer-literal> ::= $0 <number>
+  // <integer-literal> ::= <auto-nttp>
+  //
+  // <auto-nttp> ::= $ M <type> 0 <number>
   Out << "$";
 
   // Since MSVC 2019, add 'M[<type>]' after '$' for auto template parameter when
@@ -1632,8 +1684,11 @@ void MicrosoftCXXNameMangler::mangleTemplateArg(const TemplateDecl *TD,
   //                ::= <member-data-pointer>
   //                ::= <member-function-pointer>
   //                ::= $ <constant-value>
+  //                ::= $ <auto-nttp-constant-value>
   //                ::= <template-args>
   //
+  // <auto-nttp-constant-value> ::= M <type> <constant-value>
+  //
   // <constant-value> ::= 0 <number>                   # integer
   //                  ::= 1 <mangled-name>             # address of D
   //                  ::= 2 <type> <typed-constant-value>* @ # struct
@@ -1681,15 +1736,17 @@ void MicrosoftCXXNameMangler::mangleTemplateArg(const TemplateDecl *TD,
         mangleMemberFunctionPointer(
             MD->getParent()->getMostRecentNonInjectedDecl(), MD);
       } else {
-        Out << "$1?";
-        mangleName(FD);
-        mangleFunctionEncoding(FD, /*ShouldMangle=*/true);
+        mangleFunctionPointer(FD, cast<NonTypeTemplateParmDecl>(Parm),
+                              TA.getParamTypeForDecl());
       }
     } else if (TA.getParamTypeForDecl()->isRecordType()) {
       Out << "$";
       auto *TPO = cast<TemplateParamObjectDecl>(ND);
       mangleTemplateArgValue(TPO->getType().getUnqualifiedType(),
                              TPO->getValue(), TplArgKind::ClassNTTP);
+    } else if (const VarDecl *VD = dyn_cast<VarDecl>(ND)) {
+      mangleVarDecl(VD, cast<NonTypeTemplateParmDecl>(Parm),
+                    TA.getParamTypeForDecl());
     } else {
       mangle(ND, "$1?");
     }
diff --git a/clang/test/CodeGenCXX/mangle-ms-auto-templates.cpp b/clang/test/CodeGenCXX/mangle-ms-auto-templates.cpp
index c17f5f5..ff5395c 100644
--- a/clang/test/CodeGenCXX/mangle-ms-auto-templates.cpp
+++ b/clang/test/CodeGenCXX/mangle-ms-auto-templates.cpp
@@ -18,6 +18,12 @@ auto AutoFunc() {
   return a;
 }
 
+int Func();
+int Func2();
+
+int i;
+int j;
+
 void template_mangling() {
   AutoFunc<1>();
   // AFTER: call {{.*}} @"??$AutoFunc@$MH00@@YA?A?<auto>@@XZ"
@@ -44,4 +50,29 @@ void template_mangling() {
   AutoParmsTemplate<(unsigned long)1, 9223372036854775807LL> c2;
   // AFTER: call {{.*}} @"??0?$AutoParmsTemplate@$MK00$M_J0HPPPPPPPPPPPPPPP@@@QEAA@XZ"
   // BEFORE: call {{.*}} @"??0?$AutoParmsTemplate@$00$0HPPPPPPPPPPPPPPP@@@QEAA@XZ"
+
+  AutoFunc<&i>();
+  // AFTER: call {{.*}} @"??$AutoFunc@$MPEAH1?i@@3HA@@YA?A?<auto>@@XZ"
+  // BEFORE: call {{.*}} @"??$AutoFunc@$1?i@@3HA@@YA?A?<auto>@@XZ"
+
+  AutoParmTemplate<&i> auto_int_ptr;
+  // AFTER: call {{.*}} @"??0?$AutoParmTemplate@$MPEAH1?i@@3HA@@QEAA@XZ"
+  // BEFORE: call {{.*}} @"??0?$AutoParmTemplate@$1?i@@3HA@@QEAA@XZ"
+
+  AutoParmsTemplate<&i, &j> auto_int_ptrs;
+  // AFTER: call {{.*}} @"??0?$AutoParmsTemplate@$MPEAH1?i@@3HA$MPEAH1?j@@3HA@@QEAA@XZ"
+  // BEFORE: call {{.*}} @"??0?$AutoParmsTemplate@$1?i@@3HA$1?j@@3HA@@QEAA@XZ"
+
+  AutoFunc<&Func>();
+  // AFTER: call {{.*}} @"??$AutoFunc@$MP6AHXZ1?Func@@YAHXZ@@YA?A?<auto>@@XZ"
+  // BEFORE: call {{.*}} @"??$AutoFunc@$1?Func@@YAHXZ@@YA?A?<auto>@@XZ"
+
+  AutoParmTemplate<&Func> auto_func_ptr;
+  // AFTER: call {{.*}} @"??0?$AutoParmTemplate@$MP6AHXZ1?Func@@YAHXZ@@QEAA@XZ"
+  // BEFORE: call {{.*}} @"??0?$AutoParmTemplate@$1?Func@@YAHXZ@@QEAA@XZ"
+
+  AutoParmsTemplate<&Func, &Func2> auto_func_ptrs;
+  // AFTER: call {{.*}} @"??0?$AutoParmsTemplate@$MP6AHXZ1?Func@@YAHXZ$MP6AHXZ1?Func2@@YAHXZ@@QEAA@XZ"
+  // BEFORE: call {{.*}} @"??0?$AutoParmsTemplate@$1?Func@@YAHXZ$1?Func2@@YAHXZ@@QEAA@XZ"
+
 }