[clang] [Clang][Sema] Do not mark template parameters in the exception specification as used during partial ordering (PR #91534)

[Krystian Stasiowski via cfe-commits]

https://github.com/sdkrystian created https://github.com/llvm/llvm-project/pull/91534

We do not deduce template arguments from the exception specification when determining the primary template of a function template specialization or when taking the address of a function template. Therefore, this patch changes `isAtLeastAsSpecializedAs` such that we do not mark template parameters in the exception specification as 'used' during partial ordering (per [[temp.deduct.partial] p12](http://eel.is/c++draft/temp.deduct.partial#12)) to prevent the following from being ambiguous:

```cpp
template<typename T, typename U>
void f(U) noexcept(noexcept(T())); // #1

template<typename T>
void f(T*) noexcept; // #2

template<>
void f<int>(int*) noexcept; // currently ambiguous, selects #2 with this patch applied 
```

Although there is no corresponding wording in the standard, this seems to be the intended behavior given the definition of _deduction substitution loci_ in [[temp.deduct.general] p7](http://eel.is/c++draft/temp.deduct.general#7) (and EDG does the same thing). 

>From 0ef86c6bdf8fc89e771039a0e4bae88a8ebb2702 Mon Sep 17 00:00:00 2001
From: Krystian Stasiowski <sdkrystian at gmail.com>
Date: Wed, 8 May 2024 08:43:23 -0400
Subject: [PATCH] [Clang][Sema] Do not mark template parameters in the
 exception specification as used during partial ordering

---
 clang/lib/Sema/SemaTemplateDeduction.cpp      | 36 +++++++---
 .../temp.deduct/temp.deduct.partial/p3.cpp    | 72 +++++++++++++++++++
 2 files changed, 100 insertions(+), 8 deletions(-)
 create mode 100644 clang/test/CXX/temp/temp.fct.spec/temp.deduct/temp.deduct.partial/p3.cpp

diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index fe7e35d841510..c17c5838803a8 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -5453,7 +5453,7 @@ static bool isAtLeastAsSpecializedAs(Sema &S, SourceLocation Loc,
     //     is used.
     if (DeduceTemplateArgumentsByTypeMatch(
             S, TemplateParams, FD2->getType(), FD1->getType(), Info, Deduced,
-            TDF_None,
+            TDF_AllowCompatibleFunctionType,
             /*PartialOrdering=*/true) != TemplateDeductionResult::Success)
       return false;
     break;
@@ -5485,20 +5485,40 @@ static bool isAtLeastAsSpecializedAs(Sema &S, SourceLocation Loc,
   switch (TPOC) {
   case TPOC_Call:
     for (unsigned I = 0, N = Args2.size(); I != N; ++I)
-      ::MarkUsedTemplateParameters(S.Context, Args2[I], false,
-                                   TemplateParams->getDepth(),
-                                   UsedParameters);
+      ::MarkUsedTemplateParameters(S.Context, Args2[I], /*OnlyDeduced=*/false,
+                                   TemplateParams->getDepth(), UsedParameters);
     break;
 
   case TPOC_Conversion:
-    ::MarkUsedTemplateParameters(S.Context, Proto2->getReturnType(), false,
+    ::MarkUsedTemplateParameters(S.Context, Proto2->getReturnType(),
+                                 /*OnlyDeduced=*/false,
                                  TemplateParams->getDepth(), UsedParameters);
     break;
 
   case TPOC_Other:
-    ::MarkUsedTemplateParameters(S.Context, FD2->getType(), false,
-                                 TemplateParams->getDepth(),
-                                 UsedParameters);
+    // We do not deduce template arguments from the exception specification
+    // when determining the primary template of a function template
+    // specialization or when taking the address of a function template.
+    // Therefore, we do not mark template parameters in the exception
+    // specification as used during partial ordering to prevent the following
+    // from being ambiguous:
+    //
+    //   template<typename T, typename U>
+    //   void f(U) noexcept(noexcept(T())); // #1
+    //
+    //   template<typename T>
+    //   void f(T*) noexcept; // #2
+    //
+    //   template<>
+    //   void f<int>(int*) noexcept; // explicit specialization of #2
+    //
+    // Although there is no corresponding wording in the standard, this seems
+    // to be the intended behavior given the definition of
+    // 'deduction substitution loci' in [temp.deduct].
+    ::MarkUsedTemplateParameters(
+        S.Context,
+        S.Context.getFunctionTypeWithExceptionSpec(FD2->getType(), EST_None),
+        /*OnlyDeduced=*/false, TemplateParams->getDepth(), UsedParameters);
     break;
   }
 
diff --git a/clang/test/CXX/temp/temp.fct.spec/temp.deduct/temp.deduct.partial/p3.cpp b/clang/test/CXX/temp/temp.fct.spec/temp.deduct/temp.deduct.partial/p3.cpp
new file mode 100644
index 0000000000000..cc1d4ecda2ecc
--- /dev/null
+++ b/clang/test/CXX/temp/temp.fct.spec/temp.deduct/temp.deduct.partial/p3.cpp
@@ -0,0 +1,72 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+// expected-no-diagnostics
+
+template<bool B>
+struct A { };
+
+constexpr A<false> a;
+constexpr A<false> b;
+
+constexpr int* x = nullptr;
+constexpr short* y = nullptr;
+
+namespace ExplicitArgs {
+  template<typename T, typename U>
+  constexpr int f(U) noexcept(noexcept(T())) {
+    return 0;
+  }
+
+  template<typename T>
+  constexpr int f(T*) noexcept {
+    return 1;
+  }
+
+  template<>
+  constexpr int f<int>(int*) noexcept {
+    return 2;
+  }
+
+  static_assert(f<int>(1) == 0);
+  static_assert(f<short>(y) == 1);
+  static_assert(f<int>(x) == 2);
+
+  template<typename T, typename U>
+  constexpr int g(U*) noexcept(noexcept(T())) {
+    return 3;
+  }
+
+  template<typename T>
+  constexpr int g(T) noexcept {
+    return 4;
+  }
+
+  template<>
+  constexpr int g<int>(int*) noexcept {
+    return 5;
+  }
+
+  static_assert(g<int>(y) == 3);
+  static_assert(g<short>(1) == 4);
+  static_assert(g<int>(x) == 5);
+} // namespace ExplicitArgs
+
+namespace DeducedArgs {
+  template<typename T, bool B>
+  constexpr int f(T, A<B>) noexcept(B) {
+    return 0;
+  }
+
+  template<typename T, bool B>
+  constexpr int f(T*, A<B>) noexcept(B && B) {
+    return 1;
+  }
+
+  template<>
+  constexpr int f(int*, A<false>) {
+    return 2;
+  }
+
+  static_assert(f<int*>(x, a) == 0);
+  static_assert(f<short>(y, a) == 1);
+  static_assert(f<int>(x, a) == 2);
+} // namespace DeducedArgs