Fixed a recent regression that resulted in unsolved type arguments when a "bare" generic class (like dict) is passed as an argument to a function that accepts a type[T]. This addresses #5392. (#5494)

Co-authored-by: Eric Traut <erictr@microsoft.com>

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

@@ -397,8 +397,14 @@ export function assignTypeToTypeVar(
     // If the source is a class that is missing type arguments, fill
     // in missing type arguments with Unknown.
     if ((flags & AssignTypeFlags.AllowUnspecifiedTypeArguments) === 0) {
-        if (isClass(adjSrcType) && adjSrcType.includeSubclasses) {
-            adjSrcType = specializeWithDefaultTypeArgs(adjSrcType);
+        if (isClass(adjSrcType)) {
+            // Skip this if the source is a concrete class and the dest is
+            // not a type[T]. This combination is used for class decorators
+            // such as dataclass_transform, and we need to retain the original
+            // unspecialized class in this case.
+            if (adjSrcType.includeSubclasses || TypeBase.isInstantiable(destType)) {
+                adjSrcType = specializeWithDefaultTypeArgs(adjSrcType);
+            }
         }
     }
 

packages/pyright-internal/src/tests/samples/solver27.py

@@ -0,0 +1,23 @@
+# This sample tests that the assignment of an instantiable generic class
+# without supplied type arguments is given default type arguments (typically
+# Unknown) when the TypeVar is solved.
+
+from typing import Any, TypeVar, reveal_type
+
+T = TypeVar("T")
+
+
+def deco1(t: type[T], val: Any) -> T:
+    return val
+
+
+v1 = deco1(dict, {"foo": "bar"})
+reveal_type(v1, expected_text="dict[Unknown, Unknown]")
+
+
+def deco2(t: T, val: Any) -> T:
+    return val
+
+
+v2 = deco2(dict, {"foo": "bar"})
+reveal_type(v2, expected_text="type[dict[Unknown, Unknown]]")

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

@@ -653,6 +653,12 @@ test('Solver26', () => {
     TestUtils.validateResults(analysisResults, 0);
 });
 
+test('Solver27', () => {
+    const analysisResults = TestUtils.typeAnalyzeSampleFiles(['solver27.py']);
+
+    TestUtils.validateResults(analysisResults, 0);
+});
+
 test('SolverScoring1', () => {
     const analysisResults = TestUtils.typeAnalyzeSampleFiles(['solverScoring1.py']);