Fix issue 20012 - clear diagnostic for CTFE array cast in UDA context

compiler/src/dmd/dsymbolsem.d

@@ -4014,7 +4014,9 @@ private extern(C++) final class DsymbolSemanticVisitor : Visitor
         //printf("UserAttributeDeclaration::semantic() %p\n", this);
         if (uad.decl && !uad._scope)
             uad.Dsymbol.setScope(sc); // for function local symbols
-        arrayExpressionSemantic(uad.atts.peekSlice(), sc, true);
+        Scope* sc2 = sc.startCTFE();
+        arrayExpressionSemantic(uad.atts.peekSlice(), sc2, true);
+        sc2.endCTFE();
         return attribSemantic(uad);
     }
 

compiler/test/fail_compilation/fail20012.d

@@ -0,0 +1,9 @@
+/* https://github.com/dlang/dmd/issues/20012
+TEST_OUTPUT:
+---
+fail_compilation/fail20012.d(8): Error: array cast from `string` to `dstring` is not supported at compile time
+fail_compilation/fail20012.d(9): Error: array cast from `string` to `dstring` is not supported at compile time
+---
+*/
+enum a = cast(dstring) "what"c;
+@(cast(dstring) "huh"c) int x;