Merge pull request #6262 from MartinNowak/fix16574

fix Issue 16574 - forward reference issue with with speculative test

src/aggregate.d

@@ -333,8 +333,13 @@ extern (C++) abstract class AggregateDeclaration : ScopeDsymbol
         // There's unresolvable forward reference.
         if (type != Type.terror)
             error(loc, "no size because of forward reference");
-        type = Type.terror;
-        errors = true;
+        // Don't cache errors from speculative semantic, might be resolvable later.
+        // https://issues.dlang.org/show_bug.cgi?id=16574
+        if (!global.gag)
+        {
+            type = Type.terror;
+            errors = true;
+        }
         return false;
     }
 

test/compilable/test16574.d

@@ -0,0 +1,34 @@
+// https://issues.dlang.org/show_bug.cgi?id=16574
+template Recursive(T) if (is(T == class))
+{
+    // fails because T is still forward referenced
+    // speculative determineSize must not set type to error
+    static assert (!__traits(compiles, { new T; }));
+    // known size of class
+    static assert (is(typeof(T.init) == T));
+
+    alias Recursive = T;
+}
+
+// must be resolvable
+class C
+{
+    Recursive!C r;
+}
+
+template Recursive(T) if (is(T == struct))
+{
+    // fails because T is still forward referenced
+    // speculative determineSize must not set type to error
+    static assert (!__traits(compiles, { T t; }));
+    // no size yet for struct
+    static assert (!is(typeof(T.init)));
+
+    alias Recursive = T*;
+}
+
+// must be resolvable
+struct S
+{
+    Recursive!S r;
+}