Split f-string tests and fix ast visit of JoinedStr

Author: Adrien Chauve

mypy/fastparse.py

@@ -774,11 +774,14 @@ def visit_Str(self, n: ast35.Str) -> Union[UnicodeExpr, StrExpr]:
         # JoinedStr(expr* values)
         @with_line
         def visit_JoinedStr(self, n: ast35.JoinedStr) -> StrExpr:
-            return StrExpr(n.values[0])
+            result_string_expression = StrExpr('')
+            for value in n.values:
+                result_string_expression = OpExpr('+', result_string_expression, self.visit(value))
+            return result_string_expression
 
         # FormattedValue(expr value)
         @with_line
-        def visit_FormattedValue(self, n: ast35.FormattedValue) -> StrExpr:
+        def visit_FormattedValue(self, n: ast35.FormattedValue) -> Expression:
             return self.visit(n.value)
 
     # Bytes(bytes s)

test-data/unit/check-expressions.test

@@ -1162,21 +1162,23 @@ u'%s' % (u'abc',)
 -- --------
 
 
-[case testFStringParseOk]
-# flags: --fast-parser
+[case testFStringBasics]
+# flags: --fast-parser --python-version 3.6
 f'foobar'
+f'{"foobar"}'
+f'foo{"bar"}'
+a: str
+a = f'foobar'
+a = f'{"foobar"}'
 
-[case testFStringTypecheckExpression]
+[case testFStringExpressions]
 # flags: --fast-parser --python-version 3.6
-a: str
-a = 'foo'
-b: str
-b = f'{a}bar'
-b == 'foobar'
-f'{1 + "a"}'
-f'{1 + 1}'
+f'{1 + ""}'
+f' {1 + ""}'
 [out]
-main:7: error: Unsupported operand types for + ("int" and "str") (diff)
+main:2: error: Unsupported operand types for + ("int" and "str")
+main:3: error: Unsupported operand types for + ("int" and "str")
+main:3: error: Unsupported operand types for + ("str" and "int")
 
 
 -- Lambdas