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

clang/docs/ReleaseNotes.rst

@@ -683,6 +683,8 @@ Bug Fixes to C++ Support
 - Fix an assertion failure when parsing an invalid members of an anonymous class. (#GH85447)
 - Fixed a misuse of ``UnresolvedLookupExpr`` for ill-formed templated expressions. Fixes (#GH48673), (#GH63243)
   and (#GH88832).
+- Clang now defers all substitution into the exception specification of a function template specialization
+  until the noexcept-specifier is instantiated.
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaInit.cpp

@@ -6576,12 +6576,12 @@ 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 +9641,8 @@ 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 +9652,21 @@ 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?

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -1323,13 +1323,11 @@ bool Sema::isSameOrCompatibleFunctionType(QualType P, QualType A) {
     return Context.hasSameType(P, A);
 
   // Noreturn and noexcept adjustment.
-  QualType AdjustedParam;
-  if (IsFunctionConversion(P, A, AdjustedParam))
-    return Context.hasSameType(AdjustedParam, A);
+  if (QualType AdjustedParam; IsFunctionConversion(P, A, AdjustedParam))
+    P = AdjustedParam;
 
   // 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
@@ -3509,23 +3507,6 @@ 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(),
@@ -4705,13 +4686,6 @@ 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

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);

clang/test/CXX/except/except.spec/p13.cpp

@@ -72,3 +72,30 @@ template<>
 void f(A, int***); // expected-error {{'f<A, int>' is missing exception specification 'noexcept'}}
 
 }
+
+namespace N3 {
+
+template<typename T, typename U>
+void f(T, U) noexcept(T::y); // #1
+
+template<typename T, typename U> // #2
+void f(T, U*) noexcept(T::x);
+
+// Deduction should succeed for both candidates, and #2 should be selected by overload resolution.
+// Only the exception specification of #2 should be instantiated.
+void (*x)(A, int*) = f;
+}
+
+namespace N4 {
+
+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
+// expected-error@-1 {{no member named 'y' in 'A'}}
+
+// Deduction should succeed for both candidates, and #2 should be selected by overload resolution.
+// Only the exception specification of #2 should be instantiated.
+void (*x)(A, int*) = f; // expected-note {{in instantiation of exception specification for 'f<A, int>' requested here}}
+}

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}}

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);

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