[Clang][Sema] Explicit template arguments are not substituted into the exception specification of a function

[sdkrystian]

[temp.deduct.general] p6 states:

At certain points in the template argument deduction process it is necessary to take a function type that makes use of template parameters and replace those template parameters with the corresponding template arguments.
This is done at the beginning of template argument deduction when any explicitly specified template arguments are substituted into the function type, and again at the end of template argument deduction when any template arguments that were deduced or obtained from default arguments are substituted.

[temp.deduct.general] p7 goes on to say:

The deduction substitution loci are

• the function type outside of the noexcept-specifier,
• the explicit-specifier,
• the template parameter declarations, and
• the template argument list of a partial specialization

The substitution occurs in all types and expressions that are used in the deduction substitution loci. [...]

Consider the following:

struct A
{
    static constexpr bool x = true;
};

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

template<typename T, typename U>
void f(T, U*) noexcept(T::y); // #2

template<>
void f<A>(A, int*) noexcept; // clang currently accepts, GCC and EDG reject

Currently, Sema::SubstituteExplicitTemplateArguments will substitute into the noexcept-specifier when deducing template arguments from a function declaration or when deducing template arguments for taking the address of a function template (and the substitution is treated as a SFINAE context). In the above example, #1 is selected as the primary template because substitution of the explicit template arguments into the noexcept-specifier of #2 failed, which resulted in the candidate being ignored.

This behavior is incorrect (note 4 says as much), and this patch corrects it by deferring all substitution into the noexcept-specifier until it is instantiated.

As part of the necessary changes to make this patch work, the instantiation of the exception specification of a function template specialization when taking the address of a function template is changed to only occur for the function selected by overload resolution per [except.spec] p13.1 (as opposed to being instantiated for every candidate).

[llvmbot]

@llvm/pr-subscribers-clang

Author: Krystian Stasiowski (sdkrystian)

Changes

[[temp.deduct.general] p6](eel.is/c++draft/temp.deduct.general#6) states:
> At certain points in the template argument deduction process it is necessary to take a function type that makes use of template parameters and replace those template parameters with the corresponding template arguments.
This is done at the beginning of template argument deduction when any explicitly specified template arguments are substituted into the function type, and again at the end of template argument deduction when any template arguments that were deduced or obtained from default arguments are substituted.

[[temp.deduct.general] p7](eel.is/c++draft/temp.deduct.general#7) goes on to say:
> The deduction substitution loci are
> - the function type outside of the noexcept-specifier,
> - the explicit-specifier,
> - the template parameter declarations, and
> - the template argument list of a partial specialization
>
> The substitution occurs in all types and expressions that are used in the deduction substitution loci. [...]

Currently, Sema::SubstituteExplicitTemplateArguments will substitute into the noexcept-specifier when deducing template arguments from a function declaration or when deducing template arguments for taking the address of a function template (and the substitution is treated as happening in a SFINAE context). This behavior is incorrect (note 4 says as much), and this patch corrects it by deferring all substitution into the noexcept-specifier until it is instantiated.

As part of the necessary changes to make this patch work, the instantiation of the exception specification of a function template specialization when taking the address of a function template is changed to only occur for the function selected by overload resolution per [[except.spec] p13.1](http://eel.is/c++draft/except.spec#13.1) (as opposed to being instantiated for every candidate).

---

Full diff: https://github.com/llvm/llvm-project/pull/90760.diff

6 Files Affected:

• (modified) clang/lib/Sema/SemaInit.cpp (+14-6)
• (modified) clang/lib/Sema/SemaTemplateDeduction.cpp (+2-26)
• (modified) clang/test/CXX/drs/dr13xx.cpp (+5-8)
• (added) clang/test/CXX/temp/temp.deduct/p7.cpp (+14)
• (modified) clang/test/SemaCXX/cxx1z-noexcept-function-type.cpp (+2-2)
• (modified) clang/test/SemaTemplate/temp_arg_type.cpp (+4-6)

diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index 7d9eaf6720461d..2c1c7affbc13a4 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -6576,12 +6576,10 @@ void InitializationSequence::InitializeFrom(Sema &S,
 
     AddPassByIndirectCopyRestoreStep(DestType, ShouldCopy);
   } else if (ICS.isBad()) {
-    DeclAccessPair dap;
-    if (isLibstdcxxPointerReturnFalseHack(S, Entity, Initializer)) {
+    if (isLibstdcxxPointerReturnFalseHack(S, Entity, Initializer))
       AddZeroInitializationStep(Entity.getType());
-    } else if (Initializer->getType() == Context.OverloadTy &&
-               !S.ResolveAddressOfOverloadedFunction(Initializer, DestType,
-                                                     false, dap))
+    else if (DeclAccessPair Found; Initializer->getType() == Context.OverloadTy &&
+        !S.ResolveAddressOfOverloadedFunction(Initializer, DestType, /*Complain=*/false, Found))
       SetFailed(InitializationSequence::FK_AddressOfOverloadFailed);
     else if (Initializer->getType()->isFunctionType() &&
              isExprAnUnaddressableFunction(S, Initializer))
@@ -9641,6 +9639,8 @@ bool InitializationSequence::Diagnose(Sema &S,
   if (!Failed())
     return false;
 
+  QualType DestType = Entity.getType();
+
   // When we want to diagnose only one element of a braced-init-list,
   // we need to factor it out.
   Expr *OnlyArg;
@@ -9650,11 +9650,19 @@ bool InitializationSequence::Diagnose(Sema &S,
       OnlyArg = List->getInit(0);
     else
       OnlyArg = Args[0];
+
+    if (OnlyArg->getType() == S.Context.OverloadTy) {
+      DeclAccessPair Found;
+      if (FunctionDecl *FD = S.ResolveAddressOfOverloadedFunction(
+          OnlyArg, DestType.getNonReferenceType(), /*Complain=*/false, Found)) {
+        if (Expr *Resolved = S.FixOverloadedFunctionReference(OnlyArg, Found, FD).get())
+          OnlyArg = Resolved;
+      }
+    }
   }
   else
     OnlyArg = nullptr;
 
-  QualType DestType = Entity.getType();
   switch (Failure) {
   case FK_TooManyInitsForReference:
     // FIXME: Customize for the initialized entity?
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index e93f7bd842e444..fc8a645e733926 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -1231,11 +1231,11 @@ bool Sema::isSameOrCompatibleFunctionType(QualType P, QualType A) {
   // Noreturn and noexcept adjustment.
   QualType AdjustedParam;
   if (IsFunctionConversion(P, A, AdjustedParam))
-    return Context.hasSameType(AdjustedParam, A);
+    return Context.hasSameFunctionTypeIgnoringExceptionSpec(AdjustedParam, A);
 
   // FIXME: Compatible calling conventions.
 
-  return Context.hasSameType(P, A);
+  return Context.hasSameFunctionTypeIgnoringExceptionSpec(P, A);
 }
 
 /// Get the index of the first template parameter that was originally from the
@@ -3414,23 +3414,6 @@ TemplateDeductionResult Sema::SubstituteExplicitTemplateArguments(
   if (FunctionType) {
     auto EPI = Proto->getExtProtoInfo();
     EPI.ExtParameterInfos = ExtParamInfos.getPointerOrNull(ParamTypes.size());
-
-    // In C++1z onwards, exception specifications are part of the function type,
-    // so substitution into the type must also substitute into the exception
-    // specification.
-    SmallVector<QualType, 4> ExceptionStorage;
-    if (getLangOpts().CPlusPlus17 &&
-        SubstExceptionSpec(
-            Function->getLocation(), EPI.ExceptionSpec, ExceptionStorage,
-            getTemplateInstantiationArgs(
-                FunctionTemplate, nullptr, /*Final=*/true,
-                /*Innermost=*/SugaredExplicitArgumentList->asArray(),
-                /*RelativeToPrimary=*/false,
-                /*Pattern=*/nullptr,
-                /*ForConstraintInstantiation=*/false,
-                /*SkipForSpecialization=*/true)))
-      return TemplateDeductionResult::SubstitutionFailure;
-
     *FunctionType = BuildFunctionType(ResultType, ParamTypes,
                                       Function->getLocation(),
                                       Function->getDeclName(),
@@ -4610,13 +4593,6 @@ TemplateDeductionResult Sema::DeduceTemplateArguments(
                                                Info.getLocation()))
     return TemplateDeductionResult::MiscellaneousDeductionFailure;
 
-  auto *SpecializationFPT =
-      Specialization->getType()->castAs<FunctionProtoType>();
-  if (IsAddressOfFunction && getLangOpts().CPlusPlus17 &&
-      isUnresolvedExceptionSpec(SpecializationFPT->getExceptionSpecType()) &&
-      !ResolveExceptionSpec(Info.getLocation(), SpecializationFPT))
-    return TemplateDeductionResult::MiscellaneousDeductionFailure;
-
   // Adjust the exception specification of the argument to match the
   // substituted and resolved type we just formed. (Calling convention and
   // noreturn can't be dependent, so we don't actually need this for them
diff --git a/clang/test/CXX/drs/dr13xx.cpp b/clang/test/CXX/drs/dr13xx.cpp
index dad82c4e2829f0..a334b6d01acf51 100644
--- a/clang/test/CXX/drs/dr13xx.cpp
+++ b/clang/test/CXX/drs/dr13xx.cpp
@@ -281,13 +281,10 @@ namespace cwg1330 { // cwg1330: 4 c++11
   decltype(f<char>()) f2; // #cwg1330-f-char
   bool f3 = noexcept(f<float>()); /// #cwg1330-f-float
 #endif
-  // In C++17 onwards, substituting explicit template arguments into the
-  // function type substitutes into the exception specification (because it's
-  // part of the type). In earlier languages, we don't notice there's a problem
-  // until we've already started to instantiate.
   template int f<short>(); // #cwg1330-f-short
-  // since-cxx17-error@-1 {{explicit instantiation of 'f' does not refer to a function template, variable template, member function, member class, or static data member}}
-  //   since-cxx17-note@#cwg1330-f {{candidate template ignored: substitution failure [with T = short]: type 'short' cannot be used prior to '::' because it has no members}}
+  // since-cxx17-error@#cwg1330-f {{type 'short' cannot be used prior to '::' because it has no members}}
+  //   since-cxx17-note@#cwg1330-f {{in instantiation of exception specification for 'f<short>' requested here}}
+  //   since-cxx17-note@#cwg1330-f-short {{in instantiation of function template specialization 'cwg1330::f<short>' requested here}}
 
   template<typename T> struct C {
     C() throw(typename T::type); // #cwg1330-C
@@ -500,7 +497,7 @@ namespace cwg1359 { // cwg1359: 3.5
   union B { constexpr B() = default; int a; }; // #cwg1359-B
   // cxx11-17-error@-1 {{defaulted definition of default constructor cannot be marked constexpr before C++23}}
   union C { constexpr C() = default; int a, b; }; // #cwg1359-C
-  // cxx11-17-error@-1 {{defaulted definition of default constructor cannot be marked constexpr}} 
+  // cxx11-17-error@-1 {{defaulted definition of default constructor cannot be marked constexpr}}
   struct X { constexpr X() = default; union {}; };
   // since-cxx11-error@-1 {{declaration does not declare anything}}
   struct Y { constexpr Y() = default; union { int a; }; }; // #cwg1359-Y
@@ -720,7 +717,7 @@ struct A {
 } // namespace cwg1397
 
 namespace cwg1399 { // cwg1399: dup 1388
-  template<typename ...T> void f(T..., int, T...) {} // #cwg1399-f 
+  template<typename ...T> void f(T..., int, T...) {} // #cwg1399-f
   // cxx98-error@-1 {{variadic templates are a C++11 extension}}
   void g() {
     f(0);
diff --git a/clang/test/CXX/temp/temp.deduct/p7.cpp b/clang/test/CXX/temp/temp.deduct/p7.cpp
new file mode 100644
index 00000000000000..cf6d17fc51ac95
--- /dev/null
+++ b/clang/test/CXX/temp/temp.deduct/p7.cpp
@@ -0,0 +1,14 @@
+// RUN:  %clang_cc1 -verify %s
+
+struct A {
+  static constexpr bool x = true;
+};
+
+template<typename T, typename U>
+void f(T, U) noexcept(T::x);
+
+template<typename T, typename U>
+void f(T, U*) noexcept(T::y); // expected-error {{no member named 'y' in 'A'}}
+
+template<>
+void f<A>(A, int*); // expected-note {{in instantiation of exception specification}}
diff --git a/clang/test/SemaCXX/cxx1z-noexcept-function-type.cpp b/clang/test/SemaCXX/cxx1z-noexcept-function-type.cpp
index 5e56f19477d6ca..c8204c21523a37 100644
--- a/clang/test/SemaCXX/cxx1z-noexcept-function-type.cpp
+++ b/clang/test/SemaCXX/cxx1z-noexcept-function-type.cpp
@@ -18,7 +18,7 @@ template<typename A, typename B> void redecl3() throw(B); // expected-error {{do
 
 typedef int I;
 template<bool B> void redecl4(I) noexcept(B);
-template<bool B> void redecl4(I) noexcept(B); // expected-note {{could not match 'void (I) noexcept(false)' (aka 'void (int) noexcept(false)') against 'void (int) noexcept'}}
+template<bool B> void redecl4(I) noexcept(B);
 
 void (*init_with_exact_type_a)(int) noexcept = redecl4<true>;
 void (*init_with_mismatched_type_a)(int) = redecl4<true>;
@@ -27,7 +27,7 @@ using DeducedType_a = decltype(deduce_auto_from_noexcept_function_ptr_a);
 using DeducedType_a = void (*)(int) noexcept;
 
 void (*init_with_exact_type_b)(int) = redecl4<false>;
-void (*init_with_mismatched_type_b)(int) noexcept = redecl4<false>; // expected-error {{does not match required type}}
+void (*init_with_mismatched_type_b)(int) noexcept = redecl4<false>; // expected-error {{cannot initialize a variable of type}}
 auto deduce_auto_from_noexcept_function_ptr_b = redecl4<false>;
 using DeducedType_b = decltype(deduce_auto_from_noexcept_function_ptr_b);
 using DeducedType_b = void (*)(int);
diff --git a/clang/test/SemaTemplate/temp_arg_type.cpp b/clang/test/SemaTemplate/temp_arg_type.cpp
index 9069f63e0224fe..cdbcf281125efd 100644
--- a/clang/test/SemaTemplate/temp_arg_type.cpp
+++ b/clang/test/SemaTemplate/temp_arg_type.cpp
@@ -11,7 +11,7 @@ A<0> *a1; // expected-error{{template argument for template type parameter must
 A<A> *a2; // expected-error{{use of class template 'A' requires template arguments}}
 
 A<int> *a3;
-A<int()> *a4; 
+A<int()> *a4;
 A<int(float)> *a5;
 A<A<int> > *a6;
 
@@ -95,15 +95,13 @@ namespace deduce_noexcept {
   template void dep() noexcept(true); // expected-error {{does not refer to a function template}}
   template void dep() noexcept(false); // expected-error {{does not refer to a function template}}
 
-  // FIXME: It's also not clear whether this should be valid: do we substitute
-  // into the function type (including the exception specification) or not?
-  template<typename T> typename T::type1 f() noexcept(T::a);
-  template<typename T> typename T::type2 f() noexcept(T::b) {}
+  template<typename T> typename T::type1 f() noexcept(T::a); // expected-note {{candidate}}
+  template<typename T> typename T::type2 f() noexcept(T::b) {} // expected-note {{candidate}}
   struct X {
     static constexpr bool b = true;
     using type1 = void;
     using type2 = void;
   };
-  template void f<X>();
+  template void f<X>(); // expected-error {{partial ordering for explicit instantiation of 'f' is ambiguous}}
 }
 #endif

[Endilll]

Changes to DR tests look good to me.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[sdkrystian]

Ping @erichkeane

[erichkeane]

LGTM. I want @Endilll to confirm the DR test looks right though.

[Endilll]

DR tests changes LGTM

[mizvekov]

LGTM