[clang] Do not crash on "requires" after a fatal error occurred.

[adamcz]

Summary

The code would assume that SubstExpr() cannot fail on concept
specialization. This is incorret - we give up on some things after fatal
error occurred, since there's no value in doing futher work that the
user will not see anyway. In this case, this lead to crash.

The fatal error is simulated in tests with -ferror-limit=1, but this
could happen in other cases too.

Fixes https://github.com/llvm/llvm-project/issues/55401

[ilya-biryukov]

Thanks! The fix LG, just a small nitpick for the test from my side

clang/test/SemaCXX/concept-fatal-error.cpp

8

We need want to pass -verify to clang for these directives to have an effect.
The fatal error can be matched by // expected-error@* {{too...}} (maybe that's obvious, but it took me some time to figure it out first time I needed it)
And there is no need to run FileCheck after that!

[erichkeane]

This patch makes sense to me, but please clean up the test as @ilya-biryukov suggested.

[adamcz]

improved the test

clang/test/SemaCXX/concept-fatal-error.cpp

8

Ah, thanks! I knew there was a way to match fatal errors, I just didn't know what it was.

No clue what happened to -verify, I swear it was there at some point ;-)

[ilya-biryukov]

lgtm

clang/lib/Sema/SemaExprCXX.cpp

else if (ReturnTypeRequirement.isTypeConstraint()) {
     TemplateArgumentList TAL(TemplateArgumentList::OnStack, Args);
     MultiLevelTemplateArgumentList MLTAL(TAL);
     for (unsigned I = 0; I < TPL->getDepth(); ++I)
       MLTAL.addOuterRetainedLevel();
     Expr *IDC =
         cast<TemplateTypeParmDecl>(TPL->getParam(0))->getTypeConstraint()
             ->getImmediatelyDeclaredConstraint();
     ExprResult Constraint = SubstExpr(IDC, MLTAL);
-    assert(!Constraint.isInvalid() &&
-           "Substitution cannot fail as it is simply putting a type template "
-           "argument into a concept specialization expression's parameter.");
-
+    if (Constraint.isInvalid()) {
+      Status = concepts::ExprRequirement::SS_ExprSubstitutionFailure;
+    } else {
       SubstitutedConstraintExpr =
           cast<ConceptSpecializationExpr>(Constraint.get());
       if (!SubstitutedConstraintExpr->isSatisfied())
         Status = concepts::ExprRequirement::SS_ConstraintsNotSatisfied;
     }
+  }
   return new (Context) concepts::ExprRequirement(E, IsSimple, NoexceptLoc,
                                                  ReturnTypeRequirement, Status,
                                                  SubstitutedConstraintExpr);
 }
 
 concepts::ExprRequirement *
 Sema::BuildExprRequirement(
     concepts::Requirement::SubstitutionDiagnostic *ExprSubstitutionDiagnostic,

clang/test/SemaCXX/concept-fatal-error.cpp

+// RUN: not %clang_cc1 -fsyntax-only -std=c++20 -ferror-limit 1 %s 2>&1 | FileCheck %s
+
+template <class>
+concept f = requires { 42; };
+struct h {
+  // The missing semicolon will trigger an error and -ferror-limit=1 will make it fatal
+  // We test that we do not crash in such cases (#55401)
+  int i = requires { { i } f } // expected-error {{expected ';' at end of declaration list}}
+
+  // CHECK: fatal error: too many errors emitted, stopping now
+};