bpo-44792: Improve syntax errors for if expressions (GH-27506) (GH-27565)

(cherry picked from commit 28b6dc9dd5d1ce6f8aff7e06d4ef9afdc2bc8332)

Co-authored-by: Miguel Brito <5544985+miguendes@users.noreply.github.com>

1 files changed, 33 insertions, 0 deletions

diff --git a/Parser/parser.c b/Parser/parser.c
index c4a4eb6..b37e3d6 100644
--- a/Parser/parser.c
+++ b/Parser/parser.c
@@ -18229,6 +18229,7 @@ invalid_legacy_expression_rule(Parser *p)
 // invalid_expression:
 //     | invalid_legacy_expression
 //     | !(NAME STRING | SOFT_KEYWORD) disjunction expression_without_invalid
+//     | disjunction 'if' disjunction !'else'
 static void *
 invalid_expression_rule(Parser *p)
 {
@@ -18287,6 +18288,38 @@ invalid_expression_rule(Parser *p)
         D(fprintf(stderr, "%*c%s invalid_expression[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "!(NAME STRING | SOFT_KEYWORD) disjunction expression_without_invalid"));
     }
+    { // disjunction 'if' disjunction !'else'
+        if (p->error_indicator) {
+            D(p->level--);
+            return NULL;
+        }
+        D(fprintf(stderr, "%*c> invalid_expression[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "disjunction 'if' disjunction !'else'"));
+        Token * _keyword;
+        expr_ty a;
+        expr_ty b;
+        if (
+            (a = disjunction_rule(p))  // disjunction
+            &&
+            (_keyword = _PyPegen_expect_token(p, 510))  // token='if'
+            &&
+            (b = disjunction_rule(p))  // disjunction
+            &&
+            _PyPegen_lookahead_with_int(0, _PyPegen_expect_token, p, 516)  // token='else'
+        )
+        {
+            D(fprintf(stderr, "%*c+ invalid_expression[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "disjunction 'if' disjunction !'else'"));
+            _res = RAISE_SYNTAX_ERROR_KNOWN_RANGE ( a , b , "expected 'else' after 'if' expression" );
+            if (_res == NULL && PyErr_Occurred()) {
+                p->error_indicator = 1;
+                D(p->level--);
+                return NULL;
+            }
+            goto done;
+        }
+        p->mark = _mark;
+        D(fprintf(stderr, "%*c%s invalid_expression[%d-%d]: %s failed!\n", p->level, ' ',
+                  p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "disjunction 'if' disjunction !'else'"));
+    }
     _res = NULL;
   done:
     D(p->level--);