Recommit [C++20] [Modules] Trying to compare the trailing require cla…

…use from the primary template function

Close #60890.

For the following example:

```
export module a;

export template<typename T>
struct a {
	friend void aa(a) requires(true) {
	}
};
```

```
export module b;

import a;

struct b {
	a<int> m;
};
```

```
export module c;

import a;

struct c {
	void f() const {
		aa(a<int>());
	}
};
```

```
import a;
import b;
import c;

void d() {
	aa(a<int>());
}
```

The current clang will reject this incorrectly. The reason is that the
require clause  will be replaced with the evaluated version
(https://github.com/llvm/llvm-project/blob/efae3174f09560353fb0f3d528bcbffe060d5438/clang/lib/Sema/SemaConcept.cpp#L664-L665).
In module 'b', the friend function is instantiated but not used so the
require clause of the friend function is `(true)`. However, in module
'c', the friend function is used so the require clause is `true`. So
deserializer classify these two function to two different functions
instead of one. Then here is the bug report.

The proposed solution is to try to compare the trailing require clause
of the primary template when performing ODR checking.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D144626

clang/lib/AST/ASTContext.cpp

@@ -6591,7 +6591,10 @@ bool ASTContext::FriendsDifferByConstraints(const FunctionDecl *X,
 
   // If the two functions share lexical declaration context, they are not in
   // separate instantations, and thus in the same scope.
-  if (X->getLexicalDeclContext() == Y->getLexicalDeclContext())
+  if (declaresSameEntity(cast<Decl>(X->getLexicalDeclContext()
+                             ->getRedeclContext()),
+                         cast<Decl>(Y->getLexicalDeclContext()
+                             ->getRedeclContext())))
     return false;
 
   if (!X->getDescribedFunctionTemplate()) {
@@ -6683,8 +6686,28 @@ bool ASTContext::isSameEntity(const NamedDecl *X, const NamedDecl *Y) const {
         return false;
     }
 
-    if (!isSameConstraintExpr(FuncX->getTrailingRequiresClause(),
-                              FuncY->getTrailingRequiresClause()))
+    // The trailing require clause of instantiated function may change during
+    // the semantic analysis. Trying to get the primary template function (if
+    // exists) to compare the primary trailing require clause.
+    auto TryToGetPrimaryTemplatedFunction =
+        [](const FunctionDecl *FD) -> const FunctionDecl * {
+      switch (FD->getTemplatedKind()) {
+      case FunctionDecl::TK_DependentNonTemplate:
+        return FD->getInstantiatedFromDecl();
+      case FunctionDecl::TK_FunctionTemplate:
+        return FD->getDescribedFunctionTemplate()->getTemplatedDecl();
+      case FunctionDecl::TK_MemberSpecialization:
+        return FD->getInstantiatedFromMemberFunction();
+      case FunctionDecl::TK_FunctionTemplateSpecialization:
+        return FD->getPrimaryTemplate()->getTemplatedDecl();
+      default:
+        return FD;
+      }
+    };
+    const FunctionDecl *PrimaryX = TryToGetPrimaryTemplatedFunction(FuncX);
+    const FunctionDecl *PrimaryY = TryToGetPrimaryTemplatedFunction(FuncY);
+    if (!isSameConstraintExpr(PrimaryX->getTrailingRequiresClause(),
+                              PrimaryY->getTrailingRequiresClause()))
       return false;
 
     // Constrained friends are different in certain cases, see: [temp.friend]p9.

clang/test/Modules/pr60890.cppm

@@ -0,0 +1,82 @@
+// https://github.com/llvm/llvm-project/issues/60890
+//
+// 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 -emit-module-interface %t/b.cppm -fprebuilt-module-path=%t -o %t/b.pcm
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/c.cppm -fprebuilt-module-path=%t -o %t/c.pcm
+// RUN: %clang_cc1 -std=c++20 %t/d.cpp -fprebuilt-module-path=%t -S -emit-llvm -o -
+
+//--- a.cppm
+export module a;
+
+export template<typename T>
+struct a {
+	friend void aa(a x) requires(true) {}
+	void aaa() requires(true) {}
+};
+
+export template struct a<double>;
+
+export template<typename T>
+void foo(T) requires(true) {}
+
+export template void foo<double>(double);
+
+export template <typename T>
+class A {
+	friend void foo<>(A);
+};
+
+//--- b.cppm
+export module b;
+
+import a;
+
+void b() {
+    a<int> _;
+	a<double> __;
+}
+
+//--- c.cppm
+export module c;
+
+import a;
+
+struct c {
+	void f() const {
+		a<int> _;
+		aa(_);
+		_.aaa();
+
+		a<double> __;
+		aa(__);
+		__.aaa();
+
+		foo<int>(5);
+		foo<double>(3.0);
+		foo(A<int>());
+	}
+};
+
+//--- d.cpp
+// expected-no-diagnostics
+import a;
+import b;
+import c;
+
+void d() {
+	a<int> _;
+	aa(_);
+	_.aaa();
+
+	a<double> __;
+	aa(__);
+	__.aaa();
+
+	foo<int>(5);
+	foo<double>(3.0);
+	foo(A<int>());
+}