Make --pretty work better on multi-line issues

We can just print the first line. Ideally, we would print the first line, last
line, and explicitly elide the lines in between. That's for a future change!

Author: A5rocks

mypy/errors.py

@@ -1003,6 +1003,9 @@ def format_messages(
                     marker = "^"
                     if end_line == line and end_column > column:
                         marker = f'^{"~" * (end_column - column - 1)}'
+                    elif end_line != line:
+                        # just highlight the first line instead
+                        marker = f'^{"~" * (len(source_line_expanded) - column - 1)}'
                     a.append(" " * (DEFAULT_SOURCE_OFFSET + column) + marker)
         return a
 

test-data/unit/check-unreachable-code.test

@@ -1619,3 +1619,14 @@ reveal_type(bar().attr)  # N: Revealed type is "Never"
 1  # not unreachable
 reveal_type(foo().attr)  # N: Revealed type is "Never"
 1  # E: Statement is unreachable
+
+[case testUnreachableStatementPrettyHighlighting]
+# flags: --warn-unreachable --pretty
+def x() -> None:
+    assert False
+    if 5:
+        pass
+[out]
+main:4: error: Statement is unreachable
+        if 5:
+        ^~~~~