From 5571c7436e2ad87f6b6cc8fa4ecfb6653f328532 Mon Sep 17 00:00:00 2001
From: Titian Cernicova-Dragomir <dragomirtitian@yahoo.com>
Date: Sat, 22 Jun 2019 11:30:01 +0300
Subject: [PATCH] Improved parameter names for call signatures resulting from
 unions when only one parameter name is available.

---
 src/compiler/checker.ts                         | 17 ++++++++++++-----
 ...lledUnionsOfDissimilarTyeshaveGoodDisplay.ts | 12 ++++++++++++
 2 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index b5baa61172988..0dce88f89a3e1 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -7117,9 +7117,11 @@ namespace ts {
         }
 
         function combineUnionParameters(left: Signature, right: Signature) {
-            const longest = getParameterCount(left) >= getParameterCount(right) ? left : right;
+            const leftCount = getParameterCount(left);
+            const rightCount = getParameterCount(right);
+            const longest = leftCount >= rightCount ? left : right;
             const shorter = longest === left ? right : left;
-            const longestCount = getParameterCount(longest);
+            const longestCount = longest === left ? leftCount : rightCount;
             const eitherHasEffectiveRest = (hasEffectiveRestParameter(left) || hasEffectiveRestParameter(right));
             const needsExtraRestElement = eitherHasEffectiveRest && !hasEffectiveRestParameter(longest);
             const params = new Array<Symbol>(longestCount + (needsExtraRestElement ? 1 : 0));
@@ -7129,11 +7131,16 @@ namespace ts {
                 const unionParamType = getIntersectionType([longestParamType, shorterParamType]);
                 const isRestParam = eitherHasEffectiveRest && !needsExtraRestElement && i === (longestCount - 1);
                 const isOptional = i >= getMinArgumentCount(longest) && i >= getMinArgumentCount(shorter);
-                const leftName = getParameterNameAtPosition(left, i);
-                const rightName = getParameterNameAtPosition(right, i);
+                const leftName = i >= leftCount ? undefined : getParameterNameAtPosition(left, i);
+                const rightName = i >= rightCount ? undefined : getParameterNameAtPosition(right, i);
+
+                const paramName = leftName === rightName ? leftName :
+                    !leftName ? rightName :
+                    !rightName ? leftName :
+                    undefined;
                 const paramSymbol = createSymbol(
                     SymbolFlags.FunctionScopedVariable | (isOptional && !isRestParam ? SymbolFlags.Optional : 0),
-                    leftName === rightName ? leftName : `arg${i}` as __String
+                    paramName || `arg${i}` as __String
                 );
                 paramSymbol.type = isRestParam ? createArrayType(unionParamType) : unionParamType;
                 params[i] = paramSymbol;
diff --git a/tests/cases/fourslash/calledUnionsOfDissimilarTyeshaveGoodDisplay.ts b/tests/cases/fourslash/calledUnionsOfDissimilarTyeshaveGoodDisplay.ts
index e8c42871b6748..a9b10fc67adad 100644
--- a/tests/cases/fourslash/calledUnionsOfDissimilarTyeshaveGoodDisplay.ts
+++ b/tests/cases/fourslash/calledUnionsOfDissimilarTyeshaveGoodDisplay.ts
@@ -34,6 +34,14 @@
 ////    ;
 ////
 ////callableThing4(/*4*/);
+////
+////declare const callableThing5: 
+////    | (<U>(a1: U) => void)
+////    | (() => void) 
+////    ;
+////
+////callableThing5(/*5*/1)
+////
 
 verify.signatureHelp({
     marker: "1",
@@ -50,4 +58,8 @@ verify.signatureHelp({
 {
     marker: "4",
     text: "callableThing4(arg0: { x: number; } & { y: number; } & { z: number; } & { u: number; } & { v: number; } & { w: number; }): void"
+},
+{
+    marker: "5",
+    text: "callableThing5(a1: number): void"
 });