build: disallow ignoring blocking errors

Fixes python#9727 (and effectively also adds a test for python#9674)

mypy/build.py

@@ -2748,7 +2748,8 @@ def load_graph(sources: List[BuildSource], manager: BuildManager,
             manager.errors.set_file(st.xpath, st.id)
             manager.errors.report(
                 -1, -1,
-                "Duplicate module named '%s' (also at '%s')" % (st.id, graph[st.id].xpath)
+                "Duplicate module named '%s' (also at '%s')" % (st.id, graph[st.id].xpath),
+                blocker=True,
             )
             p1 = len(pathlib.PurePath(st.xpath).parents)
             p2 = len(pathlib.PurePath(graph[st.id].xpath).parents)

mypy/errors.py

@@ -316,7 +316,7 @@ def report(self,
         if end_line is None:
             end_line = origin_line
 
-        code = code or codes.MISC
+        code = code or (codes.MISC if not blocker else None)
 
         info = ErrorInfo(self.import_context(), file, self.current_module(), type,
                          function, line, column, severity, message, code,
@@ -357,14 +357,17 @@ def add_error_info(self, info: ErrorInfo) -> None:
         self._add_error_info(file, info)
 
     def is_ignored_error(self, line: int, info: ErrorInfo, ignores: Dict[int, List[str]]) -> bool:
+        if info.blocker:
+            # Blocking errors can never be ignored
+            return False
         if info.code and self.is_error_code_enabled(info.code) is False:
             return True
-        elif line not in ignores:
+        if line not in ignores:
             return False
-        elif not ignores[line]:
+        if not ignores[line]:
             # Empty list means that we ignore all errors
             return True
-        elif info.code and self.is_error_code_enabled(info.code) is True:
+        if info.code and self.is_error_code_enabled(info.code) is True:
             return info.code.code in ignores[line]
         return False
 

test-data/unit/cmdline.test

@@ -1241,3 +1241,14 @@ class Thing: ...
 [out]
 Success: no issues found in 1 source file
 == Return code: 0
+
+[case testBlocker]
+# cmd: mypy pkg --error-summary --disable-error-code syntax
+[file pkg/x.py]
+public static void main(String[] args)
+[file pkg/y.py]
+x: str = 0
+[out]
+pkg/x.py:1: error: invalid syntax
+Found 1 error in 1 file (errors prevented further checking)
+== Return code: 2