[clang] [Clang] Fix ICE in constraint normalization when substituting concept template parameters (PR #184406)

[Corentin Jabot via cfe-commits]

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@@ -4481,11 +4481,40 @@ ExprResult Sema::SubstConceptTemplateArguments(
 
     ExprResult TransformUnresolvedLookupExpr(UnresolvedLookupExpr *E,
                                              bool IsAddressOfOperand = false) {
-      if (E->isConceptReference()) {
-        ExprResult Res = SemaRef.SubstExpr(E, MLTAL);
-        return Res;
-      }
-      return E;
+      if (!E->isConceptReference())
+        return E;
+
+      assert(E->getNumDecls() == 1 &&
+             "ConceptReference must have single declaration");
+      NamedDecl *D = *E->decls_begin();
+      ConceptDecl *ResolvedConcept = nullptr;
+
+      if (auto *TTP = dyn_cast<TemplateTemplateParmDecl>(D)) {
+        unsigned Depth = TTP->getDepth();
+        unsigned Pos = TTP->getPosition();
+        if (Depth < MLTAL.getNumLevels() &&
+            MLTAL.hasTemplateArgument(Depth, Pos)) {
+          TemplateArgument Arg = MLTAL(Depth, Pos);
+          assert(Arg.getKind() == TemplateArgument::Template);
+          ResolvedConcept =
+              dyn_cast<ConceptDecl>(Arg.getAsTemplate().getAsTemplateDecl());
+        }
+        if (ResolvedConcept == nullptr)
+          return E;
+      } else
+        ResolvedConcept = dyn_cast<ConceptDecl>(D);
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cor3ntin wrote:

it can never be anything else and you were never checking if it was null

https://github.com/llvm/llvm-project/pull/184406