Reject callable assignment when dest keyword form is incompatible

packages/pyright-internal/src/analyzer/typeEvaluator.ts

@@ -28393,7 +28393,14 @@ export function createTypeEvaluator(
                         return;
                     }
 
-                    const destParamInfo = destParamMap.get(srcParamInfo.param.name);
+                    const destParamInfo =
+                        destParamMap.get(srcParamInfo.param.name) ??
+                        destParamDetails.params.find(
+                            (paramInfo) =>
+                                paramInfo.param.name === srcParamInfo.param.name &&
+                                paramInfo.kind === ParamKind.Standard &&
+                                paramInfo.param.category === ParamCategory.Simple
+                        );
                     const paramDiag = diag?.createAddendum();
                     const srcParamType = srcParamInfo.type;
 
@@ -28523,6 +28530,43 @@ export function createTypeEvaluator(
                 }
             });
 
+            // Positional-or-keyword dest parameters can also be passed by name.
+            // If the source has no matching named parameter, the keyword form must
+            // be compatible with the source **kwargs (when present).
+            destParamDetails.params.forEach((destParamInfo) => {
+                if (
+                    destParamInfo.kind !== ParamKind.Standard ||
+                    !destParamInfo.param.name ||
+                    destParamInfo.param.category !== ParamCategory.Simple
+                ) {
+                    return;
+                }
+
+                const srcHasNamed = srcParamDetails.params.some(
+                    (srcParamInfo) =>
+                        srcParamInfo.param.name === destParamInfo.param.name &&
+                        srcParamInfo.param.category === ParamCategory.Simple &&
+                        srcParamInfo.kind !== ParamKind.Positional
+                );
+                if (srcHasNamed || srcParamDetails.kwargsIndex === undefined) {
+                    return;
+                }
+
+                if (
+                    !assignParam(
+                        destParamInfo.type,
+                        srcParamDetails.params[srcParamDetails.kwargsIndex].type,
+                        destParamInfo.index,
+                        diag?.createAddendum(),
+                        constraints,
+                        flags,
+                        recursionCount
+                    )
+                ) {
+                    canAssign = false;
+                }
+            });
+
             // If both src and dest have a "**kwargs" parameter, make sure their types are compatible.
             if (srcParamDetails.kwargsIndex !== undefined && destParamDetails.kwargsIndex !== undefined) {
                 if (

packages/pyright-internal/src/tests/samples/callbackProtocol12.py

@@ -0,0 +1,37 @@
+# This sample tests that a callable with *args/**kwargs is not assignable
+# to a callable whose positional-or-keyword parameter cannot be passed
+# by keyword to the source (typing spec callable assignability).
+
+from typing import Protocol
+
+
+class AcceptsArgsKwargs(Protocol):
+    def __call__(self, *args: int, **kwargs: bool) -> None: ...
+
+
+class AcceptsKeywordOrPositional(Protocol):
+    def __call__(self, a: int) -> None: ...
+
+
+def func1(cb: AcceptsArgsKwargs):
+    # This should generate an error because AcceptsKeywordOrPositional
+    # can be called as cb(a=10), which is not valid for **kwargs: bool.
+    x: AcceptsKeywordOrPositional = cb
+
+
+class AcceptsKeywordOnly(Protocol):
+    def __call__(self, *args: int, a: bool = False) -> None: ...
+
+
+def func2(cb: AcceptsKeywordOnly):
+    # This should generate an error because the keyword form of
+    # parameter "a" has type int on the dest and bool on the source.
+    y: AcceptsKeywordOrPositional = cb
+
+
+def ok_cb(a: int) -> None:
+    pass
+
+
+def func3(cb: AcceptsKeywordOrPositional):
+    z: AcceptsKeywordOrPositional = ok_cb

packages/pyright-internal/src/tests/typeEvaluator2.test.ts

@@ -79,6 +79,12 @@ test('CallbackProtocol11', () => {
     TestUtils.validateResults(analysisResults, 0);
 });
 
+test('CallbackProtocol12', () => {
+    const analysisResults = TestUtils.typeAnalyzeSampleFiles(['callbackProtocol12.py']);
+
+    TestUtils.validateResults(analysisResults, 2);
+});
+
 test('Assignment1', () => {
     const analysisResults = TestUtils.typeAnalyzeSampleFiles(['assignment1.py']);