Remove duplicate call/construct signatures in intersections

Author: ahejlsberg

src/compiler/checker.ts

@@ -7234,8 +7234,8 @@ namespace ts {
         function resolveIntersectionTypeMembers(type: IntersectionType) {
             // The members and properties collections are empty for intersection types. To get all properties of an
             // intersection type use getPropertiesOfType (only the language service uses this).
-            let callSignatures: ReadonlyArray<Signature> = emptyArray;
-            let constructSignatures: ReadonlyArray<Signature> = emptyArray;
+            let callSignatures: Signature[] | undefined;
+            let constructSignatures: Signature[] | undefined;
             let stringIndexInfo: IndexInfo | undefined;
             let numberIndexInfo: IndexInfo | undefined;
             const types = type.types;
@@ -7257,13 +7257,22 @@ namespace ts {
                             return clone;
                         });
                     }
-                    constructSignatures = concatenate(constructSignatures, signatures);
+                    constructSignatures = appendSignatures(constructSignatures, signatures);
                 }
-                callSignatures = concatenate(callSignatures, getSignaturesOfType(t, SignatureKind.Call));
+                callSignatures = appendSignatures(callSignatures, getSignaturesOfType(t, SignatureKind.Call));
                 stringIndexInfo = intersectIndexInfos(stringIndexInfo, getIndexInfoOfType(t, IndexKind.String));
                 numberIndexInfo = intersectIndexInfos(numberIndexInfo, getIndexInfoOfType(t, IndexKind.Number));
             }
-            setStructuredTypeMembers(type, emptySymbols, callSignatures, constructSignatures, stringIndexInfo, numberIndexInfo);
+            setStructuredTypeMembers(type, emptySymbols, callSignatures || emptyArray, constructSignatures || emptyArray, stringIndexInfo, numberIndexInfo);
+        }
+
+        function appendSignatures(signatures: Signature[] | undefined, newSignatures: readonly Signature[]) {
+            for (const sig of newSignatures) {
+                if (!signatures || every(signatures, s => !compareSignaturesIdentical(s, sig, /*partialMatch*/ false, /*ignoreThisTypes*/ false, /*ignoreReturnTypes*/ false, compareTypesIdentical))) {
+                    signatures = append(signatures, sig);
+                }
+            }
+            return signatures;
         }
 
         /**