Fix checking first member of union after zero size field

compiler/src/dmd/dstruct.d

@@ -233,8 +233,12 @@ extern (C++) class StructDeclaration : AggregateDeclaration
         auto lastOffset = -1;
         foreach (vd; fields)
         {
+            // First skip zero sized fields
+            if (vd.type.size(vd.loc) == 0)
+                continue;
+
             // only consider first sized member of an (anonymous) union
-            if (vd.overlapped && vd.offset == lastOffset && vd.type.size(vd.loc) != 0)
+            if (vd.overlapped && vd.offset == lastOffset)
                 continue;
             lastOffset = vd.offset;
 
@@ -246,10 +250,6 @@ extern (C++) class StructDeclaration : AggregateDeclaration
                      */
                     continue;
 
-                // Zero size fields are zero initialized
-                if (vd.type.size(vd.loc) == 0)
-                    continue;
-
                 // Examine init to see if it is all 0s.
                 auto exp = vd.getConstInitializer();
                 if (!exp || !_isZeroInit(exp))

compiler/test/compilable/isZeroInit.d

@@ -103,7 +103,6 @@ union U
     float x = 0;
     float y;
 }
-
 static assert(__traits(isZeroInit, U));
 
 union U2
@@ -111,5 +110,13 @@ union U2
     float x;
     int y;
 }
-
 static assert(!__traits(isZeroInit, U2));
+
+struct S8 {
+    int[0] dummy; // same offset as anon union, but doesn't overlap; should be ignored anyway for zero-init check
+    union {
+        float f; // is the first member of the anon union and must be checked
+        int i;
+    }
+}
+static assert(!__traits(isZeroInit, S8));