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

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

Differential Revision: https://reviews.llvm.org/D129499

clang/lib/Sema/SemaExprCXX.cpp

@@ -9006,14 +9006,14 @@ Sema::BuildExprRequirement(
         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.");
-
-    SubstitutedConstraintExpr =
-        cast<ConceptSpecializationExpr>(Constraint.get());
-    if (!SubstitutedConstraintExpr->isSatisfied())
-      Status = concepts::ExprRequirement::SS_ConstraintsNotSatisfied;
+    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,

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

@@ -0,0 +1,10 @@
+// RUN: not %clang_cc1 -fsyntax-only -std=c++20 -ferror-limit 1 -verify %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}}
+                               // expected-error@* {{too many errros emitted}}
+};