Added exemption to the reportUnnecessaryComparison rule for simple …

…wildcard patterns in a `case` clause. This addresses #4706. (#9582)
microsoft · Dec 13, 2024 · 9041f88 · 9041f88
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commit 9041f88
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diff --git a/docs/configuration.md b/docs/configuration.md
@@ -199,7 +199,7 @@ The following settings allow more fine grained control over the **typeCheckingMo
 
 - <a name="reportUnnecessaryCast"></a> **reportUnnecessaryCast** [boolean or string, optional]: Generate or suppress diagnostics for `cast` calls that are statically determined to be unnecessary. Such calls are sometimes indicative of a programming error. The default value for this setting is `"none"`.
 
-- <a name="reportUnnecessaryComparison"></a> **reportUnnecessaryComparison** [boolean or string, optional]: Generate or suppress diagnostics for `==` or `!=` comparisons or other conditional expressions that are statically determined to always evaluate to False or True. Such comparisons are sometimes indicative of a programming error. The default value for this setting is `"none"`.
+- <a name="reportUnnecessaryComparison"></a> **reportUnnecessaryComparison** [boolean or string, optional]: Generate or suppress diagnostics for `==` or `!=` comparisons or other conditional expressions that are statically determined to always evaluate to False or True. Such comparisons are sometimes indicative of a programming error. The default value for this setting is `"none"`. Also reports `case` clauses in a `match` statement that can be statically determined to never match (with exception of the `_` wildcard pattern, which is exempted).
 
 - <a name="reportUnnecessaryContains"></a> **reportUnnecessaryContains** [boolean or string, optional]: Generate or suppress diagnostics for `in` operations that are statically determined to always evaluate to False or True. Such operations are sometimes indicative of a programming error. The default value for this setting is `"none"`.
 

diff --git a/packages/pyright-internal/src/analyzer/patternMatching.ts b/packages/pyright-internal/src/analyzer/patternMatching.ts
@@ -2174,6 +2174,16 @@ export function getPatternSubtypeNarrowingCallback(
 }
 
 function reportUnnecessaryPattern(evaluator: TypeEvaluator, pattern: PatternAtomNode, subjectType: Type): void {
+    // If this is a simple wildcard pattern, exempt it from this diagnostic.
+    if (
+        pattern.nodeType === ParseNodeType.PatternAs &&
+        pattern.d.orPatterns.length === 1 &&
+        pattern.d.orPatterns[0].nodeType === ParseNodeType.PatternCapture &&
+        pattern.d.orPatterns[0].d.isWildcard
+    ) {
+        return;
+    }
+
     evaluator.addDiagnostic(
         DiagnosticRule.reportUnnecessaryComparison,
         LocMessage.patternNeverMatches().format({ type: evaluator.printType(subjectType) }),

diff --git a/packages/pyright-internal/src/tests/samples/matchUnnecessary1.py b/packages/pyright-internal/src/tests/samples/matchUnnecessary1.py
@@ -84,3 +84,14 @@ def func4(vals: list[str]) -> TA1:
             case _:
                 pass
     return x
+
+
+def func5(subj: int | str):
+    match subj:
+        case int() | str():
+            pass
+
+        # This should not generate a diagnostic becuase _ is exempted
+        # from the reportUnnecessaryComparison check.
+        case _:
+            pass