From 5bf8f53afa8a9085a5c08deed9c1418c12ccc6d8 Mon Sep 17 00:00:00 2001
From: Brian Schubert <brianm.schubert@gmail.com>
Date: Wed, 20 Aug 2025 18:25:20 -0400
Subject: [PATCH] Fail gracefully on unsupported template strings (PEP 750)

---
 mypy/fastparse.py                   | 22 ++++++++++++++++++++++
 mypy/test/testcheck.py              |  2 ++
 mypy/test/testparse.py              |  4 ++++
 test-data/unit/check-python314.test |  3 +++
 test-data/unit/parse-python314.test |  5 +++++
 5 files changed, 36 insertions(+)
 create mode 100644 test-data/unit/check-python314.test
 create mode 100644 test-data/unit/parse-python314.test

diff --git a/mypy/fastparse.py b/mypy/fastparse.py
index 0e1b66f0db59..99d5c48c92d7 100644
--- a/mypy/fastparse.py
+++ b/mypy/fastparse.py
@@ -188,6 +188,13 @@ def ast3_parse(
     ast_TypeVar = Any
     ast_TypeVarTuple = Any
 
+if sys.version_info >= (3, 14):
+    ast_TemplateStr = ast3.TemplateStr
+    ast_Interpolation = ast3.Interpolation
+else:
+    ast_TemplateStr = Any
+    ast_Interpolation = Any
+
 N = TypeVar("N", bound=Node)
 
 # There is no way to create reasonable fallbacks at this stage,
@@ -1705,6 +1712,21 @@ def visit_FormattedValue(self, n: ast3.FormattedValue) -> Expression:
         )
         return self.set_line(result_expression, n)
 
+    # TemplateStr(expr* values)
+    def visit_TemplateStr(self, n: ast_TemplateStr) -> Expression:
+        self.fail(
+            ErrorMessage("PEP 750 template strings are not yet supported"),
+            n.lineno,
+            n.col_offset,
+            blocker=False,
+        )
+        e = TempNode(AnyType(TypeOfAny.from_error))
+        return self.set_line(e, n)
+
+    # Interpolation(expr value, constant str, int conversion, expr? format_spec)
+    def visit_Interpolation(self, n: ast_Interpolation) -> Expression:
+        assert False, "Unreachable"
+
     # Attribute(expr value, identifier attr, expr_context ctx)
     def visit_Attribute(self, n: Attribute) -> MemberExpr | SuperExpr:
         value = n.value
diff --git a/mypy/test/testcheck.py b/mypy/test/testcheck.py
index 206eab726a21..04ef5370d381 100644
--- a/mypy/test/testcheck.py
+++ b/mypy/test/testcheck.py
@@ -47,6 +47,8 @@
     typecheck_files.remove("check-python312.test")
 if sys.version_info < (3, 13):
     typecheck_files.remove("check-python313.test")
+if sys.version_info < (3, 14):
+    typecheck_files.remove("check-python314.test")
 
 
 class TypeCheckSuite(DataSuite):
diff --git a/mypy/test/testparse.py b/mypy/test/testparse.py
index 027ca4dd2887..c8bcb5c0d807 100644
--- a/mypy/test/testparse.py
+++ b/mypy/test/testparse.py
@@ -27,6 +27,8 @@ class ParserSuite(DataSuite):
         files.remove("parse-python312.test")
     if sys.version_info < (3, 13):
         files.remove("parse-python313.test")
+    if sys.version_info < (3, 14):
+        files.remove("parse-python314.test")
 
     def run_case(self, testcase: DataDrivenTestCase) -> None:
         test_parser(testcase)
@@ -46,6 +48,8 @@ def test_parser(testcase: DataDrivenTestCase) -> None:
         options.python_version = (3, 12)
     elif testcase.file.endswith("python313.test"):
         options.python_version = (3, 13)
+    elif testcase.file.endswith("python314.test"):
+        options.python_version = (3, 14)
     else:
         options.python_version = defaults.PYTHON3_VERSION
 
diff --git a/test-data/unit/check-python314.test b/test-data/unit/check-python314.test
new file mode 100644
index 000000000000..f1043aab860a
--- /dev/null
+++ b/test-data/unit/check-python314.test
@@ -0,0 +1,3 @@
+[case testTemplateString]
+reveal_type(t"mypy")  # E: PEP 750 template strings are not yet supported \
+                      # N: Revealed type is "Any"
diff --git a/test-data/unit/parse-python314.test b/test-data/unit/parse-python314.test
new file mode 100644
index 000000000000..34fe753084f6
--- /dev/null
+++ b/test-data/unit/parse-python314.test
@@ -0,0 +1,5 @@
+[case testTemplateString]
+x = 'mypy'
+t'Hello {x}'
+[out]
+main:2: error: PEP 750 template strings are not yet supported