51134: ! return doesn't change the return status

Src/exec.c

@@ -1961,7 +1961,7 @@ execpline(Estate state, wordcode slcode, int how, int last1)
 	}
 	else
 	    unqueue_signals();
-	if ((slflags & WC_SUBLIST_NOT) && !errflag)
+	if ((slflags & WC_SUBLIST_NOT) && !errflag && !retflag)
 	    lastval = !lastval;
     }
     if (!pline_level)

Test/A01grammar.ztst

@@ -970,3 +970,15 @@ F:its expectations.
 0:Non-interactive shell command input is line buffered
 >Value is first
 >Value is second
+
+ fn() {
+   ! false
+ }
+0:! inverts the status of implicit return
+
+ fn () {
+   false
+   ! return
+ }
+ fn
+1:! does not affect return status of explicit return