bpo-1875: Raise SyntaxError in invalid blocks that will be optimised away (GH-13332)

Move the check for dead conditionals (if 0) to the peephole optimizer
and make sure that the code block is still compiled to report any
existing syntax errors within.

2 files changed, 16 insertions, 9 deletions

diff --git a/Python/compile.c b/Python/compile.c
index 91ce04b02e..2a086a509f 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -2546,13 +2546,12 @@ compiler_if(struct compiler *c, stmt_ty s)
         return 0;
 
     constant = expr_constant(s->v.If.test);
-    /* constant = 0: "if 0"
+    /* constant = 0: "if 0" Leave the optimizations to
+     * the pephole optimizer to check for syntax errors
+     * in the block.
      * constant = 1: "if 1", "if 2", ...
      * constant = -1: rest */
-    if (constant == 0) {
-        if (s->v.If.orelse)
-            VISIT_SEQ(c, stmt, s->v.If.orelse);
-    } else if (constant == 1) {
+    if (constant == 1) {
         VISIT_SEQ(c, stmt, s->v.If.body);
     } else {
         if (asdl_seq_LEN(s->v.If.orelse)) {
diff --git a/Python/peephole.c b/Python/peephole.c
index cc244aa433..1ce3535626 100644
--- a/Python/peephole.c
+++ b/Python/peephole.c
@@ -302,11 +302,19 @@ PyCode_Optimize(PyObject *code, PyObject* consts, PyObject *names,
             case LOAD_CONST:
                 cumlc = lastlc + 1;
                 if (nextop != POP_JUMP_IF_FALSE  ||
-                    !ISBASICBLOCK(blocks, op_start, i + 1)  ||
-                    !PyObject_IsTrue(PyList_GET_ITEM(consts, get_arg(codestr, i))))
+                    !ISBASICBLOCK(blocks, op_start, i + 1)) {
                     break;
-                fill_nops(codestr, op_start, nexti + 1);
-                cumlc = 0;
+                }
+                PyObject* cnt = PyList_GET_ITEM(consts, get_arg(codestr, i));
+                int is_true = PyObject_IsTrue(cnt);
+                if (is_true == 1) {
+                    fill_nops(codestr, op_start, nexti + 1);
+                    cumlc = 0;
+                } else if (is_true == 0) {
+                    h = get_arg(codestr, nexti) / sizeof(_Py_CODEUNIT);
+                    tgt = find_op(codestr, codelen, h);
+                    fill_nops(codestr, op_start, tgt);
+                }
                 break;
 
                 /* Try to fold tuples of constants.