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

[via cfe-commits]

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@@ -4481,11 +4481,39 @@ 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;
+
+      NamedDecl *D = *E->decls_begin();
+      ConceptDecl *ResolvedConcept = nullptr;
+
+      if (const auto * const TTP = dyn_cast<TemplateTemplateParmDecl>(D)) {
+        const auto Depth = TTP->getDepth();
+        const auto Pos = TTP->getPosition();
+        if (Depth < MLTAL.getNumLevels() &&
+            MLTAL.hasTemplateArgument(Depth, Pos)) {
+          TemplateArgument Arg = MLTAL(Depth, Pos);
+          if (Arg.getKind() == TemplateArgument::Template)
+            ResolvedConcept = dyn_cast_or_null<ConceptDecl>(
+                Arg.getAsTemplate().getAsTemplateDecl());
+        }
+      } else
+        ResolvedConcept = dyn_cast<ConceptDecl>(D);
+
+      if (ResolvedConcept == nullptr)
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wx257osn2 wrote:

no, `dyn_cast_or_null<ConceptDecl>(Arg.getAsTemplate().getAsTemplateDecl())` can be null. however it doesn't on `else` branch, so moved it into the `if` branch

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