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[Clang][Sema] Defer instantiation of exception specification until after partial ordering when determining primary template (llvm#82417)
Consider the following: ``` struct A { static constexpr bool x = true; }; template<typename T, typename U> void f(T, U) noexcept(T::y); // #1, error: no member named 'y' in 'A' template<typename T, typename U> void f(T, U*) noexcept(T::x); // #2 template<> void f(A, int*) noexcept; // explicit specialization of #2 ``` We currently instantiate the exception specification of all candidate function template specializations when deducting template arguments for an explicit specialization, which results in a error despite `#1` not being selected by partial ordering as the most specialized template. According to [except.spec] p13: > An exception specification is considered to be needed when: > - [...] > - the exception specification is compared to that of another declaration (e.g., an explicit specialization or an overriding virtual function); Assuming that "comparing declarations" means "determining whether the declarations correspond and declare the same entity" (per [basic.scope.scope] p4 and [basic.link] p11.1, respectively), the exception specification does _not_ need to be instantiated until _after_ partial ordering, at which point we determine whether the implicitly instantiated specialization and the explicit specialization declare the same entity (the determination of whether two functions/function templates correspond does not consider the exception specifications). This patch defers the instantiation of the exception specification until a single function template specialization is selected via partial ordering, matching the behavior of GCC, EDG, and MSVC: see https://godbolt.org/z/Ebb6GTcWE.
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‎clang/docs/ReleaseNotes.rst‎

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@@ -281,6 +281,8 @@ Bug Fixes to C++ Support
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a requires-clause lie at the same depth as those of the surrounding lambda. This,
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in turn, results in the wrong template argument substitution during constraint checking.
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(`#78524 <https://github.com/llvm/llvm-project/issues/78524>`_)
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- Clang no longer instantiates the exception specification of discarded candidate function
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templates when determining the primary template of an explicit specialization.
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Bug Fixes to AST Handling
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^^^^^^^^^^^^^^^^^^^^^^^^^

‎clang/lib/Sema/SemaTemplate.cpp‎

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@@ -9709,6 +9709,40 @@ bool Sema::CheckFunctionTemplateSpecialization(
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// Ignore access information; it doesn't figure into redeclaration checking.
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FunctionDecl *Specialization = cast<FunctionDecl>(*Result);
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// C++23 [except.spec]p13:
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// An exception specification is considered to be needed when:
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// - [...]
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// - the exception specification is compared to that of another declaration
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// (e.g., an explicit specialization or an overriding virtual function);
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// - [...]
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//
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// The exception specification of a defaulted function is evaluated as
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// described above only when needed; similarly, the noexcept-specifier of a
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// specialization of a function template or member function of a class
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// template is instantiated only when needed.
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//
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// The standard doesn't specify what the "comparison with another declaration"
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// entails, nor the exact circumstances in which it occurs. Moreover, it does
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// not state which properties of an explicit specialization must match the
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// primary template.
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//
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// We assume that an explicit specialization must correspond with (per
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// [basic.scope.scope]p4) and declare the same entity as (per [basic.link]p8)
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// the declaration produced by substitution into the function template.
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//
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// Since the determination whether two function declarations correspond does
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// not consider exception specification, we only need to instantiate it once
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// we determine the primary template when comparing types per
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// [basic.link]p11.1.
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auto *SpecializationFPT =
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Specialization->getType()->castAs<FunctionProtoType>();
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// If the function has a dependent exception specification, resolve it after
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// we have selected the primary template so we can check whether it matches.
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if (getLangOpts().CPlusPlus17 &&
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isUnresolvedExceptionSpec(SpecializationFPT->getExceptionSpecType()) &&
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!ResolveExceptionSpec(FD->getLocation(), SpecializationFPT))
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return true;
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FunctionTemplateSpecializationInfo *SpecInfo
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= Specialization->getTemplateSpecializationInfo();
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assert(SpecInfo && "Function template specialization info missing?");

‎clang/lib/Sema/SemaTemplateDeduction.cpp‎

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@@ -4632,11 +4632,9 @@ TemplateDeductionResult Sema::DeduceTemplateArguments(
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Info.getLocation()))
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return TemplateDeductionResult::MiscellaneousDeductionFailure;
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// If the function has a dependent exception specification, resolve it now,
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// so we can check that the exception specification matches.
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auto *SpecializationFPT =
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Specialization->getType()->castAs<FunctionProtoType>();
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if (getLangOpts().CPlusPlus17 &&
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if (IsAddressOfFunction && getLangOpts().CPlusPlus17 &&
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isUnresolvedExceptionSpec(SpecializationFPT->getExceptionSpecType()) &&
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!ResolveExceptionSpec(Info.getLocation(), SpecializationFPT))
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return TemplateDeductionResult::MiscellaneousDeductionFailure;
@@ -4662,11 +4660,11 @@ TemplateDeductionResult Sema::DeduceTemplateArguments(
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// specialization with respect to arguments of compatible pointer to function
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// types, template argument deduction fails.
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if (!ArgFunctionType.isNull()) {
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if (IsAddressOfFunction
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? !isSameOrCompatibleFunctionType(
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Context.getCanonicalType(SpecializationType),
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Context.getCanonicalType(ArgFunctionType))
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: !Context.hasSameType(SpecializationType, ArgFunctionType)) {
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if (IsAddressOfFunction ? !isSameOrCompatibleFunctionType(
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Context.getCanonicalType(SpecializationType),
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Context.getCanonicalType(ArgFunctionType))
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: !Context.hasSameFunctionTypeIgnoringExceptionSpec(
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SpecializationType, ArgFunctionType)) {
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Info.FirstArg = TemplateArgument(SpecializationType);
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Info.SecondArg = TemplateArgument(ArgFunctionType);
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return TemplateDeductionResult::NonDeducedMismatch;

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

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// RUN: %clang_cc1 -fexceptions -fcxx-exceptions -fsyntax-only -verify %s
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struct A {
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static constexpr bool x = true;
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};
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namespace N0 {
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template<typename T, typename U>
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void f(T, U) noexcept(T::y); // #1
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template<typename T, typename U> // #2
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void f(T, U*) noexcept(T::x);
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// Deduction should succeed for both candidates, and #2 should be selected as the primary template.
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// Only the exception specification of #2 should be instantiated.
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template<>
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void f(A, int*) noexcept;
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}
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namespace N1 {
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template<typename T, typename U>
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void f(T, U) noexcept(T::x); // #1
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template<typename T, typename U>
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void f(T, U*) noexcept(T::y); // #2
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// expected-error@-1 {{no member named 'y' in 'A'}}
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// Deduction should succeed for both candidates, and #2 should be selected as the primary template.
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// Only the exception specification of #2 should be instantiated.
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template<>
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void f(A, int*) noexcept; // expected-error {{exception specification in declaration does not match previous declaration}}
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// expected-note@-1 {{in instantiation of exception specification for 'f<A, int>' requested here}}
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// expected-note@-2 {{previous declaration is here}}
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}
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namespace N2 {
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template<typename T, typename U>
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void f(T, U) noexcept(T::x);
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template<typename T, typename U>
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void f(T, U*) noexcept(T::x);
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template<typename T, typename U>
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void f(T, U**) noexcept(T::y); // expected-error {{no member named 'y' in 'A'}}
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template<typename T, typename U>
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void f(T, U***) noexcept(T::x);
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template<>
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void f(A, int*) noexcept; // expected-note {{previous declaration is here}}
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template<>
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void f(A, int*); // expected-error {{'f<A, int>' is missing exception specification 'noexcept'}}
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template<>
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void f(A, int**) noexcept; // expected-error {{exception specification in declaration does not match previous declaration}}
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// expected-note@-1 {{in instantiation of exception specification for 'f<A, int>' requested here}}
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// expected-note@-2 {{previous declaration is here}}
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// FIXME: Exception specification is currently set to EST_None if instantiation fails.
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template<>
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void f(A, int**);
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template<>
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void f(A, int***) noexcept; // expected-note {{previous declaration is here}}
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template<>
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void f(A, int***); // expected-error {{'f<A, int>' is missing exception specification 'noexcept'}}
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}

‎clang/test/SemaTemplate/class-template-noexcept.cpp‎

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@@ -2,9 +2,7 @@
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// RUN: %clang_cc1 -std=c++11 -verify %s
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// RUN: %clang_cc1 -std=c++17 -verify %s
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// RUN: %clang_cc1 -std=c++1z -verify %s
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#if __cplusplus >= 201703
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// expected-no-diagnostics
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#endif
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class A {
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public:
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static const char X;
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template<typename U> void func() noexcept(U::X);
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template<class... B, char x>
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#if __cplusplus >= 201703
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void foo(void(B...) noexcept(x)) {}
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#else
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void foo(void(B...) noexcept(x)) {} // expected-note{{candidate template ignored}}
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#endif
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void bar()
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{
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#if __cplusplus >= 201703
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foo(func<A>);
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#else
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foo(func<A>); // expected-error{{no matching function for call}}
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#endif
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}
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