printf: Print special error for invalid octal numbers

(tbh these were always a mistake)

See #9035

src/builtins/printf.cpp

@@ -169,6 +169,12 @@ void builtin_printf_state_t::verify_numeric(const wchar_t *s, const wchar_t *end
         } else {
             // This isn't entirely fatal - the value should still be printed.
             this->nonfatal_error(_(L"%ls: value not completely converted (can't convert '%ls')"), s, end);
+            // Warn about octal numbers as they can be confusing.
+            // Do it if the unconverted digit is a valid hex digit,
+            // because it could also be an "0x" -> "0" typo.
+            if (*s == L'0' && iswxdigit(*end)) {
+                this->nonfatal_error(_(L"Hint: a leading '0' without an 'x' indicates an octal number"), s, end);
+            }
         }
     }
 }

tests/checks/printf.fish

@@ -95,3 +95,27 @@ printf '%d\n' 15.1
 # CHECKERR: 15.1: value not completely converted (can't convert '.1')
 echo $status
 # CHECK: 1
+
+printf '%d\n' 07
+# CHECK: 7
+echo $status
+# CHECK: 0
+printf '%d\n' 08
+# CHECK: 0
+# CHECKERR: 08: value not completely converted (can't convert '8')
+# CHECKERR: Hint: a leading '0' without an 'x' indicates an octal number
+echo $status
+# CHECK: 1
+
+printf '%d\n' 0f
+# CHECK: 0
+# CHECKERR: 0f: value not completely converted (can't convert 'f')
+# CHECKERR: Hint: a leading '0' without an 'x' indicates an octal number
+echo $status
+# CHECK: 1
+
+printf '%d\n' 0g
+# CHECK: 0
+# CHECKERR: 0g: value not completely converted (can't convert 'g')
+echo $status
+# CHECK: 1