[Clang][Sema] ASTContext::getUnconstrainedType propagates dependence

[sdkrystian]

When the argument passed to ASTContext::getUnconstrainedType is an unconstrained AutoType, will return the argument unchanged. However, when called with a constrained AutoType, an unconstrained, non-dependent AutoType will be returned even if the argument was dependent. Consider the following:

template<typename T>
concept C = sizeof(T) == sizeof(int);

template<auto N>
struct A;

template<C auto N>
struct A<N>; // error: class template partial specialization is not more specialized than the primary template

When comparing the template parameters for equivalence, ASTContext::getUnconstrainedType is used to remove the constraints per [temp.over.link] p6 sentence 2. For the template parameter N of the class template, it returns a dependent AutoType. For the template parameter N of the class template partial specialization, it returns a non-dependent AutoType. We subsequently compare the adjusted types and find they are not equivalent, thus we consider the partial specialization to not be more specialized than the primary template per [temp.func.order] p6.2.2.

This patch changes ASTContext::getUnconstrainedType such that the dependence of a constrained AutoType will propagate to the returned unconstrained AutoType. This causes the above example to be correctly accepted, fixing #77377.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Krystian Stasiowski (sdkrystian)

Changes

When the argument passed to ASTContext::getUnconstrainedType is an unconstrained AutoType, will return the argument unchanged. However, when called with a constrained AutoType, an unconstrained, non-dependent AutoType will be returned even if the argument was dependent. Consider the following:

template<typename T>
concept C = sizeof(T) == sizeof(int);

template<auto N>
struct A;

template<C auto N>
struct A<N> { }; // error: class template partial specialization is not more specialized than the primary template

When comparing the template parameters for equivalence, ASTContext::getUnconstrainedType is used to remove the constraints per [[temp.over.link] p6 sentence 2](http://eel.is/c++draft/temp.over.link#6.sentence-2). For the template parameter N of the class template, it returns a dependent AutoType. For the template parameter N of the class template partial specialization, it returns a non-dependent AutoType. We subsequently compare the adjusted types and find they are not equivalent, thus we consider the partial specialization to not be more specialized than the primary template per [[temp.func.order] p6.2.2](http://eel.is/c++draft/temp.func.order#6.2.2).

This patch changes ASTContext::getUnconstrainedType such that the dependence of a constrained AutoType will propagate to the returned unconstrained AutoType. This causes the above example to be correctly accepted, fixing #77377.

---

Full diff: https://github.com/llvm/llvm-project/pull/92425.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+1)
• (modified) clang/lib/AST/ASTContext.cpp (+2-1)
• (added) clang/test/CXX/temp/temp.decls/temp.fct/temp.func.order/p2.cpp (+11)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index be4cded276321..21f273cf8f54e 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -723,6 +723,7 @@ Bug Fixes to C++ Support
 - Clang now ignores template parameters only used within the exception specification of candidate function
   templates during partial ordering when deducing template arguments from a function declaration or when
   taking the address of a function template.
+- Fix a bug with checking constrained non-type template parameters for equivalence.
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index 4475f399a120b..8fc2bb8c401c2 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -5910,7 +5910,8 @@ QualType ASTContext::getUnconstrainedType(QualType T) const {
   if (auto *AT = CanonT->getAs<AutoType>()) {
     if (!AT->isConstrained())
       return T;
-    return getQualifiedType(getAutoType(QualType(), AT->getKeyword(), false,
+    return getQualifiedType(getAutoType(QualType(), AT->getKeyword(),
+                                        AT->isDependentType(),
                                         AT->containsUnexpandedParameterPack()),
                             T.getQualifiers());
   }
diff --git a/clang/test/CXX/temp/temp.decls/temp.fct/temp.func.order/p2.cpp b/clang/test/CXX/temp/temp.decls/temp.fct/temp.func.order/p2.cpp
new file mode 100644
index 0000000000000..5c908d789d511
--- /dev/null
+++ b/clang/test/CXX/temp/temp.decls/temp.fct/temp.func.order/p2.cpp
@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
+// expected-no-diagnostics
+
+template<typename T>
+concept C = sizeof(T) == sizeof(int);
+
+template<auto N>
+struct A;
+
+template<C auto N>
+struct A<N> { };

[zyn0217]

Thanks!

[shafik]

LGTM