[clang] [Clang] enhance diagnostic by attaching source location to deduced type in trailing return without auto (PR #115786)

[Mariya Podchishchaeva via cfe-commits]

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@@ -4887,9 +4887,18 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
                       cast<AutoType>(T)->getKeyword() !=
                           AutoTypeKeyword::Auto ||
                       cast<AutoType>(T)->isConstrained())) {
-            S.Diag(D.getDeclSpec().getTypeSpecTypeLoc(),
-                   diag::err_trailing_return_without_auto)
-                << T << D.getDeclSpec().getSourceRange();
+            SourceLocation Loc = D.getDeclSpec().getTypeSpecTypeLoc();
+            SourceRange SR = D.getDeclSpec().getSourceRange();
+            if (Loc.isInvalid()) {
+              TypeSourceInfo *TSI = nullptr;
+              S.GetTypeFromParser(FTI.getTrailingReturnType(), &TSI);
+              if (TSI) {
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Fznamznon wrote:

Can TSI actually be `nullptr` here? It seems we're under `FTI.hasTrailingReturnType()` branch, I would expect it to be always here.

https://github.com/llvm/llvm-project/pull/115786