[clang] [clang] more useful error message for decomposition declaration missing initializer (PR #127924)

[Erich Keane via cfe-commits]

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@@ -14058,7 +14058,17 @@ void Sema::ActOnUninitializedDecl(Decl *RealDecl) {
 
     // C++1z [dcl.dcl]p1 grammar implies that an initializer is mandatory.
     if (isa<DecompositionDecl>(RealDecl)) {
-      Diag(Var->getLocation(), diag::err_decomp_decl_requires_init) << Var;
+      Preprocessor &PP = getPreprocessor();
+      SourceManager &SM = Context.getSourceManager();
+      LangOptions LO = Context.getLangOpts();
+
+      // Lexer previously checked for '=' and didn't find it
+      // Highlight the token found in its place in the error message
+      Token Tok;
+      Lexer::getRawToken(PP.getLastCachedTokenLocation(), Tok, SM, LO);
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erichkeane wrote:

This whole bit here with the raw-token isn't right, and shouldn't be in Sema.  We also can't count on the preprocessors 'last location' with templates, since we'll be at end of TU.

Same with the `getSpelling` part.  If we want to improve this diagnostic, we need tog et the information from the AST somehow.

https://github.com/llvm/llvm-project/pull/127924