[clang] [Clang] Only compare template params of potential overload after checking their decl context (PR #78139)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-modules

Author: cor3ntin (cor3ntin)

<details>
<summary>Changes</summary>



Fixes a regression from 69066ab3 in which we compared the template lists of potential overloads before checkings their declaration contexts.

This would cause a crash when doing constraint substitution as part of that template check, because we would try to refer to not yet instantiated entities (the underlying cause is unclear).

This patch reorders (again) when we look at template parameter so we don't when checkings friends in different lexical contexts.

Fixes #<!-- -->77953
Fixes #<!-- -->78101

---
Full diff: https://github.com/llvm/llvm-project/pull/78139.diff


3 Files Affected:

- (modified) clang/lib/Sema/SemaOverload.cpp (+34-26) 
- (modified) clang/test/CXX/over/over.load/p2-0x.cpp (+27-5) 
- (added) clang/test/Modules/GH77953.cpp (+28) 


``````````diff
diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index 23b9bc0fe2d6e2..4b84a60d4780b5 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -1259,6 +1259,40 @@ static bool IsOverloadOrOverrideImpl(Sema &SemaRef, FunctionDecl *New,
   if ((OldTemplate == nullptr) != (NewTemplate == nullptr))
     return true;
 
+  // Is the function New an overload of the function Old?
+  QualType OldQType = SemaRef.Context.getCanonicalType(Old->getType());
+  QualType NewQType = SemaRef.Context.getCanonicalType(New->getType());
+
+  // Compare the signatures (C++ 1.3.10) of the two functions to
+  // determine whether they are overloads. If we find any mismatch
+  // in the signature, they are overloads.
+
+  // If either of these functions is a K&R-style function (no
+  // prototype), then we consider them to have matching signatures.
+  if (isa<FunctionNoProtoType>(OldQType.getTypePtr()) ||
+      isa<FunctionNoProtoType>(NewQType.getTypePtr()))
+    return false;
+
+  const FunctionProtoType *OldType = cast<FunctionProtoType>(OldQType);
+  const FunctionProtoType *NewType = cast<FunctionProtoType>(NewQType);
+
+  // The signature of a function includes the types of its
+  // parameters (C++ 1.3.10), which includes the presence or absence
+  // of the ellipsis; see C++ DR 357).
+  if (OldQType != NewQType && OldType->isVariadic() != NewType->isVariadic())
+    return true;
+
+  // For member-like friends, the enclosing class is part of the signature.
+  if ((New->isMemberLikeConstrainedFriend() ||
+       Old->isMemberLikeConstrainedFriend()) &&
+      !New->getLexicalDeclContext()->Equals(Old->getLexicalDeclContext()))
+    return true;
+
+  // Compare the parameter lists.
+  // This can only be done once we have establish that friend functions
+  // inhabit the same context, otherwise we might tried to instantiate
+  // references to non-instantiated entities during constraint substitution.
+  // GH78101.
   if (NewTemplate) {
     // C++ [temp.over.link]p4:
     //   The signature of a function template consists of its function
@@ -1296,34 +1330,8 @@ static bool IsOverloadOrOverrideImpl(Sema &SemaRef, FunctionDecl *New,
       return true;
   }
 
-  // Is the function New an overload of the function Old?
-  QualType OldQType = SemaRef.Context.getCanonicalType(Old->getType());
-  QualType NewQType = SemaRef.Context.getCanonicalType(New->getType());
-
-  // Compare the signatures (C++ 1.3.10) of the two functions to
-  // determine whether they are overloads. If we find any mismatch
-  // in the signature, they are overloads.
 
-  // If either of these functions is a K&R-style function (no
-  // prototype), then we consider them to have matching signatures.
-  if (isa<FunctionNoProtoType>(OldQType.getTypePtr()) ||
-      isa<FunctionNoProtoType>(NewQType.getTypePtr()))
-    return false;
 
-  const FunctionProtoType *OldType = cast<FunctionProtoType>(OldQType);
-  const FunctionProtoType *NewType = cast<FunctionProtoType>(NewQType);
-
-  // The signature of a function includes the types of its
-  // parameters (C++ 1.3.10), which includes the presence or absence
-  // of the ellipsis; see C++ DR 357).
-  if (OldQType != NewQType && OldType->isVariadic() != NewType->isVariadic())
-    return true;
-
-  // For member-like friends, the enclosing class is part of the signature.
-  if ((New->isMemberLikeConstrainedFriend() ||
-       Old->isMemberLikeConstrainedFriend()) &&
-      !New->getLexicalDeclContext()->Equals(Old->getLexicalDeclContext()))
-    return true;
   const auto *OldMethod = dyn_cast<CXXMethodDecl>(Old);
   const auto *NewMethod = dyn_cast<CXXMethodDecl>(New);
 
diff --git a/clang/test/CXX/over/over.load/p2-0x.cpp b/clang/test/CXX/over/over.load/p2-0x.cpp
index 8fd9a1ce1e87ab..a86e2d2a1d81e5 100644
--- a/clang/test/CXX/over/over.load/p2-0x.cpp
+++ b/clang/test/CXX/over/over.load/p2-0x.cpp
@@ -24,11 +24,6 @@ class Y {
   void k() &&; // expected-error{{cannot overload a member function with ref-qualifier '&&' with a member function without a ref-qualifier}}
 };
 
-struct GH76358 {
-    template<int> void f() && {}
-    template<typename T> void f() const {}
-};
-
 
 #if __cplusplus >= 202002L
 namespace GH58962 {
@@ -55,4 +50,31 @@ static_assert(not test<type<2>&>);
 static_assert(test<type<2>&&>);
 
 }
+
+namespace GH78101 {
+
+template<typename T, typename U, int i>
+concept True = true;
+
+template<typename T, int I>
+struct Template {
+    static constexpr int i = I;
+    friend constexpr auto operator+(True<T, i> auto f) {
+        return i;
+    }
+};
+
+template<int I>
+struct Template<float, I> {
+    static constexpr int i = I;
+    friend constexpr auto operator+(True<float, i> auto f) {
+        return i;
+    }
+};
+
+Template<void, 4> f{};
+static_assert(+Template<float, 5>{} == 5);
+
+}
+
 #endif
diff --git a/clang/test/Modules/GH77953.cpp b/clang/test/Modules/GH77953.cpp
new file mode 100644
index 00000000000000..aaca8153c3d212
--- /dev/null
+++ b/clang/test/Modules/GH77953.cpp
@@ -0,0 +1,28 @@
+// From https://github.com/llvm/llvm-project/issues/77953
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/a.cppm -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 -fmodule-file=a=%t/a.pcm %t/b.cppm
+
+//--- a.cppm
+export module a;
+
+template<typename, typename>
+concept c = true;
+
+export template<typename... Ts>
+struct a {
+	template<typename... Us> requires(... and c<Ts, Us>)
+	friend bool operator==(a, a<Us...>) {
+		return true;
+	}
+};
+
+template struct a<>;
+
+//--- b.cppm
+import a;
+
+template struct a<int>;

``````````

</details>


https://github.com/llvm/llvm-project/pull/78139