Add new f-string test to check expression type checking

Author: Adrien Chauve

mypy/fastparse.py

@@ -776,6 +776,12 @@ def visit_Str(self, n: ast35.Str) -> Union[UnicodeExpr, StrExpr]:
         def visit_JoinedStr(self, n: ast35.JoinedStr) -> StrExpr:
             return StrExpr(n.values[0])
 
+        # FormattedValue(expr value)
+        @with_line
+        def visit_FormattedValue(self, n: ast35.FormattedValue) -> StrExpr:
+            print(n, n.value, dir(n))
+            return self.visit(n.value)
+
     # Bytes(bytes s)
     @with_line
     def visit_Bytes(self, n: ast35.Bytes) -> Union[BytesExpr, StrExpr]:

test-data/unit/check-expressions.test

@@ -1166,6 +1166,19 @@ u'%s' % (u'abc',)
 # flags: --fast-parser
 f'foobar'
 
+[case testFStringTypecheckExpression]
+# flags: --fast-parser
+# flags: --python-version 3.6
+a: str
+a = 'foo'
+b: str
+b = f'{a}bar'
+b == 'foobar'
+f'{1 + "a"}'
+f'{1 + 1}'
+[out]
+main:2: error: Unsupported operand types for + ("int" and "str")
+
 
 -- Lambdas
 -- -------