diff --git a/compiler/src/dmd/dsymbolsem.d b/compiler/src/dmd/dsymbolsem.d
index 2e8225108b6c..dfea05ec9474 100644
--- a/compiler/src/dmd/dsymbolsem.d
+++ b/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);
     }
 
diff --git a/compiler/test/fail_compilation/fail20012.d b/compiler/test/fail_compilation/fail20012.d
new file mode 100644
index 000000000000..f30a88c15d80
--- /dev/null
+++ b/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;