[clang][Parser] Fix an assertion during decltype error recovery with missing ; (#188123)

Fix an assertion failure in `ParseDecltypeSpecifier` when parsing
malformed expressions e.g. `decltype(union { ... )`.

When a class/union definition is missing a semicolon, clang's error
recovery may synthetically set the current token to `tok::semi` without
actually inserting it into the preprocessor's backtrack cache, see
9096c9cda3/clang/lib/Parse/ParseDeclCXX.cpp (L1920-L1927)

If `ParseDecltypeSpecifier` later encounters this synthetic semicolon
during its own error recovery, its attempts to revert the cache and
re-lex the tokens will lead to reading unexpected tokens from the
historical stream, failing the strict
`Tok.is(tok::semi)` assertion.

This patch removes this assertion, acknowledging that during invalid
parses and error recovery, the preprocessor's cache state may not
strictly align with the parser's simulated state.


Fixes: #188014

clang/docs/ReleaseNotes.rst

@@ -449,6 +449,7 @@ Miscellaneous Clang Crashes Fixed
 - Fixed a crash when evaluating ``__is_bitwise_cloneable`` on invalid record types. (#GH183707)
 - Fixed an assertion failure when casting a function pointer with a target with a non-default program address space. (#GH186210)
 - Fixed a crash when ``decltype(__builtin_FUNCTION())`` is used as a template type argument. (#GH167433)
+- Fixed an assertion failure when parsing an invalid ``decltype`` specifier with missing parentheses or extra semicolons. (#GH188014)
 
 OpenACC Specific Changes
 ------------------------

clang/lib/Parse/ParseDeclCXX.cpp

@@ -1082,7 +1082,6 @@ SourceLocation Parser::ParseDecltypeSpecifier(DeclSpec &DS) {
             PP.RevertCachedTokens(2);
             ConsumeToken(); // the semi.
             EndLoc = ConsumeAnyToken();
-            assert(Tok.is(tok::semi));
           } else {
             EndLoc = Tok.getLocation();
           }

clang/test/Parser/decltype-gh188014.cpp

@@ -0,0 +1,4 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
+
+// GH188014
+& decltype ( ( union { // expected-error {{expected ';' after union}} expected-error {{expected '}'}} expected-error {{expected ')'}} expected-error {{expected expression}} expected-error {{expected unqualified-id}} expected-note {{to match this '('}} expected-note {{to match this '{'}}