[clang] [Clang] Reapply CWG2369 "Ordering between constraints and substitution" (PR #122423)

[via cfe-commits]

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@@ -5284,8 +5284,22 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
     savedContext.pop();
   }
 
-  DeclGroupRef DG(Function);
-  Consumer.HandleTopLevelDecl(DG);
+  // With CWG2369, we substitute constraints before instantiating the associated
+  // function template. This helps prevent potential code generation for
+  // dependent types, particularly under the MS ABI.
+  bool ShouldSkipCG = [&] {
+    auto *RD = dyn_cast<CXXRecordDecl>(Function->getParent());
+    if (!RD || !RD->isLambda())
+      return false;
+
+    return llvm::any_of(ExprEvalContexts, [](auto &Context) {
+      return Context.isUnevaluated() || Context.isImmediateFunctionContext();
+    });
+  }();
+  if (!ShouldSkipCG) {
+    DeclGroupRef DG(Function);
+    Consumer.HandleTopLevelDecl(DG);
+  }
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cor3ntin wrote:

This should fix https://github.com/llvm/llvm-project/issues/82926 and https://github.com/llvm/llvm-project/issues/123854 

@zyn0217 Would you be willing to make a separate PR?

(I haven't tested though)

https://github.com/llvm/llvm-project/pull/122423