[Clang][Sema] Expose static inline functions from GMF (#104701)

In C, it is a common pattern to have `static inline` functions in
headers to avoid ODR issues. Currently, when those headers are included
in a GMF, the names are not found when two-phase name lookup and ADL is
involved. Those names are removed by `Sema::AddOverloadCandidate`.

Similarly, in C++, sometimes people use templates with internal linkage
in headers.

As the GMF was designed to be a transitional mechanism for headers,
special case those functions in `Sema::AddOverloadCandidate`.

This fixes <https://github.com/llvm/llvm-project/issues/98021>.

clang/lib/Sema/SemaOverload.cpp

@@ -6977,11 +6977,26 @@ void Sema::AddOverloadCandidate(
     /// have linkage. So that all entities of the same should share one
     /// linkage. But in clang, different entities of the same could have
     /// different linkage.
-    NamedDecl *ND = Function;
-    if (auto *SpecInfo = Function->getTemplateSpecializationInfo())
+    const NamedDecl *ND = Function;
+    bool IsImplicitlyInstantiated = false;
+    if (auto *SpecInfo = Function->getTemplateSpecializationInfo()) {
       ND = SpecInfo->getTemplate();
+      IsImplicitlyInstantiated = SpecInfo->getTemplateSpecializationKind() ==
+                                 TSK_ImplicitInstantiation;
+    }
 
-    if (ND->getFormalLinkage() == Linkage::Internal) {
+    /// Don't remove inline functions with internal linkage from the overload
+    /// set if they are declared in a GMF, in violation of C++ [basic.link]p17.
+    /// However:
+    /// - Inline functions with internal linkage are a common pattern in
+    ///   headers to avoid ODR issues.
+    /// - The global module is meant to be a transition mechanism for C and C++
+    ///   headers, and the current rules as written work against that goal.
+    const bool IsInlineFunctionInGMF =
+        Function->isFromGlobalModule() &&
+        (IsImplicitlyInstantiated || Function->isInlined());
+
+    if (ND->getFormalLinkage() == Linkage::Internal && !IsInlineFunctionInGMF) {
       Candidate.Viable = false;
       Candidate.FailureKind = ovl_fail_module_mismatched;
       return;

clang/test/Modules/expose-static-inline-from-gmf-1.cppm

@@ -0,0 +1,37 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang -std=c++20 %t/a.cppm --precompile -o %t/a.pcm \
+// RUN:   -DTEST_INLINE
+// RUN: %clang -std=c++20 %t/test.cc -fprebuilt-module-path=%t -fsyntax-only -Xclang -verify \
+// RUN:   -DTEST_INLINE
+//
+// RUN: %clang -std=c++20 %t/a.cppm --precompile -o %t/a.pcm
+// RUN: %clang -std=c++20 %t/test.cc -fprebuilt-module-path=%t -fsyntax-only -Xclang -verify
+
+//--- a.h
+#ifdef TEST_INLINE
+#define INLINE inline
+#else
+#define INLINE
+#endif
+static INLINE void func(long) {}
+template <typename T = long> void a() { func(T{}); }
+
+//--- a.cppm
+module;
+#include "a.h"
+export module a;
+export using ::a;
+
+//--- test.cc
+import a;
+auto m = (a(), 0);
+
+#ifdef TEST_INLINE
+// expected-no-diagnostics
+#else
+// expected-error@a.h:7 {{no matching function for call to 'func'}}
+// expected-note@test.cc:2 {{in instantiation of function template specialization 'a<long>' requested here}}
+#endif

clang/test/Modules/expose-static-inline-from-gmf-2.cppm

@@ -0,0 +1,22 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang -std=c++20 %t/a.cppm --precompile -o %t/a.pcm
+// RUN: %clang -std=c++20 %t/test.cc -fprebuilt-module-path=%t -fsyntax-only -Xclang -verify
+
+//--- a.h
+template <typename G> static inline void func() {}
+template <typename T = long> void a() { func<T>(); }
+
+//--- a.cppm
+module;
+#include "a.h"
+export module a;
+export using ::a;
+
+//--- test.cc
+import a;
+auto m = (a(), 0);
+
+// expected-no-diagnostics

clang/test/Modules/expose-static-inline-from-gmf-3.cppm

@@ -0,0 +1,24 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang -std=c++20 %t/a.cppm --precompile -o %t/a.pcm
+// RUN: %clang -std=c++20 %t/test.cc -fprebuilt-module-path=%t -fsyntax-only -Xclang -verify
+
+//--- a.h
+namespace ns {
+template <typename G> static void func() {}
+template <typename T = long> void a() { func<T>(); }
+}
+
+//--- a.cppm
+module;
+#include "a.h"
+export module a;
+export using ns::a;
+
+//--- test.cc
+import a;
+auto m = (a(), 0);
+
+// expected-no-diagnostics

clang/test/Modules/expose-static-inline-from-gmf-4.cppm

@@ -0,0 +1,40 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang -std=c++20 %t/a.cppm --precompile -o %t/a.pcm \
+// RUN:   -DTEST_INLINE
+// RUN: %clang -std=c++20 %t/test.cc -fprebuilt-module-path=%t -fsyntax-only -Xclang -verify \
+// RUN:   -DTEST_INLINE
+//
+// RUN: %clang -std=c++20 %t/a.cppm --precompile -o %t/a.pcm
+// RUN: %clang -std=c++20 %t/test.cc -fprebuilt-module-path=%t -fsyntax-only -Xclang -verify
+
+//--- a.h
+#ifdef TEST_INLINE
+#define INLINE inline
+#else
+#define INLINE
+#endif
+namespace ns {
+template <typename G> static void func() {}
+template <> INLINE void func<long>() {}
+template <typename T = long> void a() { func<T>(); }
+}
+
+//--- a.cppm
+module;
+#include "a.h"
+export module a;
+export using ns::a;
+
+//--- test.cc
+import a;
+auto m = (a(), 0);
+
+#ifdef TEST_INLINE
+// expected-no-diagnostics
+#else
+// expected-error@a.h:9 {{no matching function for call to 'func'}}
+// expected-note@test.cc:2 {{in instantiation of function template specialization 'ns::a<long>' requested here}}
+#endif

clang/test/Modules/expose-static-inline-from-gmf-5.cppm

@@ -0,0 +1,26 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang -std=c++20 %t/a.cppm --precompile -o %t/a.pcm
+// RUN: %clang -std=c++20 %t/test.cc -fprebuilt-module-path=%t -fsyntax-only -Xclang -verify
+
+//--- a.h
+namespace ns {
+namespace {
+template <typename G> void func() {}
+}
+template <typename T = long> void a() { func<T>(); }
+}
+
+//--- a.cppm
+module;
+#include "a.h"
+export module a;
+export using ns::a;
+
+//--- test.cc
+import a;
+auto m = (a(), 0);
+
+// expected-no-diagnostics