Fix local extern vardecl

see testcases.  A local 'extern int i' declaration needs to
refer to the global declaration, not to a local one it might
be shadowing.  Doesn't seem to happen in the wild very often as
this was broken forever.

tccgen.c

@@ -995,7 +995,9 @@ static Sym *external_sym(int v, CType *type, int r, AttributeDef *ad)
 {
     Sym *s;
     s = sym_find(v);
-    if (!s) {
+    if (!s || (!(s->type.t & VT_EXTERN) && (s->type.t & VT_BTYPE) != VT_FUNC)) {
+        if (s && !is_compatible_types(&s->type, type))
+            tcc_error("conflicting types for '%s'", get_tok_str(s->v, NULL));
         /* push forward reference */
         s = sym_push(v, type, r | VT_CONST | VT_SYM, 0);
         s->type.t |= VT_EXTERN;

tests/tcctest.c

@@ -77,6 +77,7 @@ void expr_test();
 void macro_test();
 void recursive_macro_test();
 void scope_test();
+void scope_test2();
 void forward_test();
 void funcptr_test();
 void loop_test();
@@ -718,6 +719,7 @@ int main(int argc, char **argv)
     macro_test();
     recursive_macro_test();
     scope_test();
+    scope_test2();
     forward_test();
     funcptr_test();
     loop_test();
@@ -800,6 +802,20 @@ void scope_test()
     printf("g5=%d\n", g);
 }
 
+int st2_i;
+int *st2_p = &st2_i;
+void scope_test2()
+{
+    char a[50];
+    st2_i = 42;
+    for (int st2_i = 1; st2_i < 10; st2_i++) {
+        extern int st2_i;
+        st2_i++;
+        printf("exloc: %d\n", st2_i);
+    }
+    printf("exloc: %d\n", *st2_p);
+}
+
 void array_test()
 {
     int i, j, a[4];

tests/tests2/60_errors_and_warnings.c

@@ -151,4 +151,13 @@ int ga = 0.42 { 2 };
 struct S { int a, b; };
 struct T { struct S x; };
 struct T gt = { 42 a: 1, 43 };
+#elif defined test_conflicting_types
+int i;
+void foo(void) {
+    int i;
+      {
+        extern double i;
+        i = 42.2;
+      }
+}
 #endif

tests/tests2/60_errors_and_warnings.expect

@@ -71,3 +71,6 @@
 
 [test_invalid_3]
 60_errors_and_warnings.c:153: error: ',' expected (got "a")
+
+[test_conflicting_types]
+60_errors_and_warnings.c:159: error: conflicting types for 'i'