[OpenMP] Fix crash with invalid argument to simd collapse (#139313)

Same as with other recent crash fixes, this is checking whether the
argument expression contains errors or not.

Fixes #138493

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -920,6 +920,8 @@ OpenMP Support
   ``partial`` was an invalid expression. (#GH139267)
 - Fixed a crashing bug with ``omp tile sizes`` if the argument to ``sizes`` was
   an invalid expression. (#GH139073)
+- Fixed a crashing bug with ``omp simd collapse`` if the argument to
+  ``collapse`` was an invalid expression. (#GH138493)
 - Fixed a crashing bug with ``omp distribute dist_schedule`` if the argument to
   ``dist_schedule`` was not strictly positive. (#GH139266)
 

clang/lib/Sema/SemaOpenMP.cpp

@@ -9648,6 +9648,13 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
                 DSAStackTy &DSA,
                 SemaOpenMP::VarsWithInheritedDSAType &VarsWithImplicitDSA,
                 OMPLoopBasedDirective::HelperExprs &Built) {
+  // If either of the loop expressions exist and contain errors, we bail out
+  // early because diagnostics have already been emitted and we can't reliably
+  // check more about the loop.
+  if ((CollapseLoopCountExpr && CollapseLoopCountExpr->containsErrors()) ||
+      (OrderedLoopCountExpr && OrderedLoopCountExpr->containsErrors()))
+    return 0;
+
   unsigned NestedLoopCount = 1;
   bool SupportsNonPerfectlyNested = (SemaRef.LangOpts.OpenMP >= 50) &&
                                     !isOpenMPLoopTransformationDirective(DKind);

clang/test/OpenMP/simd_collapse_messages.cpp

@@ -97,3 +97,12 @@ int main(int argc, char **argv) {
   return tmain<int, char, 1, 0>(argc, argv);
 }
 
+namespace GH138493 {
+void f(void) {
+  // This would previously crash when processing an invalid expression as an
+  // argument to collapse.
+#pragma omp simd collapse(a) // expected-error {{use of undeclared identifier 'a'}}
+  for (int i = 0; i < 10; i++)
+    ;
+}
+} // namespace GH138493