[clang] [clang][Modules] Keep current-TU global-module-fragment internal-linkage functions viable in overload resolution (PR #210825)

[David Brittain via cfe-commits]

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@@ -7388,7 +7388,24 @@ void Sema::AddOverloadCandidate(
         Function->isFromGlobalModule() &&
         (IsImplicitlyInstantiated || Function->isInlined());
 
-    if (ND->getFormalLinkage() == Linkage::Internal && !IsInlineFunctionInGMF) {
+    /// An internal-linkage function declared in the global module fragment of
+    /// the module unit currently being compiled is usable here
+    /// ([module.global.frag]p1), even a plain 'static' (non-inline) helper.
+    /// Such a helper is dropped here only because
+    /// Function->isInAnotherModuleUnit() misclassifies it when the enclosing
+    /// template is instantiated after the GMF is closed: that check consults
+    /// ASTContext's current *named* module, which is not in scope yet while the
+    /// GMF's pending instantiations run. Comparing against the current module
+    /// scope instead recognises it as belonging to this unit. Only genuinely
+    /// cross-unit uses stay subject to the internal-linkage rule.
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combinatorial wrote:

The comment has been shortened to a single sentence.

https://github.com/llvm/llvm-project/pull/210825