[clang] Diagnose the code with trailing comma in the function call. (PR #125232)

[Haojian Wu via cfe-commits]

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@@ -2237,6 +2237,9 @@ Parser::ParsePostfixExpressionSuffix(ExprResult LHS) {
             if (PP.isCodeCompletionReached() && !CalledSignatureHelp)
               RunSignatureHelp();
             LHS = ExprError();
+          } else if (!HasError && HasTrailingComma) {
+            // FIXME: add a FIXIT to remove the trailing comma.
+            Diag(Tok, diag::err_extraneous_trailing_comma);
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hokein wrote:

Reused the existing err_expected_expression.

I initially thought that the new `err_extraneous_trailing_comma` provided a clearer message than `err_expected_expression`, but it only applies to a specific test case (the one added in this PR).

For other cases, such as `function2Arg(1, );`, the new diagnostic is incorrect.

https://github.com/llvm/llvm-project/pull/125232