[clang] [C++20][Modules] Load function body from the module that gives canonical decl (PR #111992)

[Dmitry Polukhin via cfe-commits]

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@@ -1976,14 +1976,16 @@ TemplateDeclInstantiator::VisitFunctionTemplateDecl(FunctionTemplateDecl *D) {
   if (!InstParams)
     return nullptr;
 
+  // Use canonical templated decl because only canonical decl has body
+  // if declarations were merged during loading from modules.
+  FunctionDecl *TemplatedDecl = D->getTemplatedDecl()->getCanonicalDecl();
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dmpolukhin wrote:

`FunctionDecl::hasBody` is doing what you described for `getRedeclWithDef`, see https://github.com/llvm/llvm-project/blob/main/clang/lib/AST/Decl.cpp#L3163 and if I used it here or implement similar code here, it fails several clang tests. It is very unfortunate that `Redeclarable` is used for two things: real redeclaration and AST merges. Moreover it look like there is no way to distinguish these two cases. I need to use different decl only if the decl was `doesThisDeclarationHaveABody() == true` but it is no longer the case due to AST merge.

https://github.com/llvm/llvm-project/pull/111992