fix Issue 15138 - ICE with basic use of stdx.data.json

src/dtemplate.d

@@ -6352,18 +6352,28 @@ public:
             // Elide codegen because this is really speculative.
             return false;
         }
+
         /* Even when this is reached to the codegen pass,
          * a non-root nested template should not generate code,
          * due to avoid ODR violation.
          */
         if (enclosing && enclosing.inNonRoot())
         {
             if (tinst)
-                return tinst.needsCodegen();
+            {
+                auto r = tinst.needsCodegen();
+                minst = tinst.minst; // cache result
+                return r;
+            }
             if (tnext)
-                return tnext.needsCodegen();
+            {
+                auto r = tnext.needsCodegen();
+                minst = tnext.minst; // cache result
+                return r;
+            }
             return false;
         }
+
         /* The issue is that if the importee is compiled with a different -debug
          * setting than the importer, the importer may believe it exists
          * in the compiled importee when it does not, when the instantiation

test/runnable/ice15138.d

@@ -0,0 +1,11 @@
+// EXTRA_SOURCES: imports/ice15138a.d
+// PERMUTE_ARGS: -unittest -inline
+// COMPILE_SEPARATELY
+
+import imports.ice15138a;
+
+void main()
+{
+    JSONValue v;
+    v.get!JSONValue;
+}

test/runnable/imports/ice15138a.d

@@ -0,0 +1,28 @@
+module imports.ice15138a;
+
+alias AliasSeq(TL...) = TL;
+
+alias FieldNameTuple(T...) = AliasSeq!();
+
+struct TaggedAlgebraic(U)
+{
+    alias X = FieldNameTuple!(U.tupleof);
+}
+
+void get(T, U)(TaggedAlgebraic!U ta) {}
+
+union PayloadUnion
+{
+    int dummy;
+}
+
+struct JSONValue
+{
+    alias Payload = TaggedAlgebraic!PayloadUnion;
+
+    void get(T)()
+    {
+        Payload payload;
+        .get!T(payload);
+    }
+}