In pickLongestCandidateSignature, instantiate using inferred type arguments

src/compiler/checker.ts

@@ -18636,7 +18636,7 @@ namespace ts {
             return getInferredTypes(context);
         }
 
-        function inferTypeArguments(node: CallLikeExpression, signature: Signature, args: ReadonlyArray<Expression>, excludeArgument: boolean[] | undefined, context: InferenceContext): Type[] {
+        function inferTypeArguments(node: CallLikeExpression, signature: Signature, args: ReadonlyArray<Expression>, excludeArgument: ReadonlyArray<boolean> | undefined, context: InferenceContext): Type[] {
             // Clear out all the inference results from the last time inferTypeArguments was called on this context
             for (const inference of context.inferences) {
                 // As an optimization, we don't have to clear (and later recompute) inferred types
@@ -19050,19 +19050,7 @@ namespace ts {
             // For a decorator, no arguments are susceptible to contextual typing due to the fact
             // decorators are applied to a declaration by the emitter, and not to an expression.
             const isSingleNonGenericCandidate = candidates.length === 1 && !candidates[0].typeParameters;
-            let excludeArgument: boolean[] | undefined;
-            if (!isDecorator && !isSingleNonGenericCandidate) {
-                // We do not need to call `getEffectiveArgumentCount` here as it only
-                // applies when calculating the number of arguments for a decorator.
-                for (let i = 0; i < args.length; i++) {
-                    if (isContextSensitive(args[i])) {
-                        if (!excludeArgument) {
-                            excludeArgument = new Array(args.length);
-                        }
-                        excludeArgument[i] = true;
-                    }
-                }
-            }
+            let excludeArgument = !isDecorator && !isSingleNonGenericCandidate ? getExcludeArgument(args) : undefined;
 
             // The following variables are captured and modified by calls to chooseOverload.
             // If overload resolution or type argument inference fails, we want to report the
@@ -19225,6 +19213,21 @@ namespace ts {
             }
         }
 
+        function getExcludeArgument(args: ReadonlyArray<Expression>): boolean[] | undefined {
+            let excludeArgument: boolean[] | undefined;
+            // We do not need to call `getEffectiveArgumentCount` here as it only
+            // applies when calculating the number of arguments for a decorator.
+            for (let i = 0; i < args.length; i++) {
+                if (isContextSensitive(args[i])) {
+                    if (!excludeArgument) {
+                        excludeArgument = new Array(args.length);
+                    }
+                    excludeArgument[i] = true;
+                }
+            }
+            return excludeArgument;
+        }
+
         // No signature was applicable. We have already reported the errors for the invalid signature.
         // If this is a type resolution session, e.g. Language Service, try to get better information than anySignature.
         function getCandidateForOverloadFailure(
@@ -19303,17 +19306,29 @@ namespace ts {
                 return candidate;
             }
 
-            const typeArgumentNodes: ReadonlyArray<TypeNode> = callLikeExpressionMayHaveTypeArguments(node) ? node.typeArguments || emptyArray : emptyArray;
+            const typeArgumentNodes: ReadonlyArray<TypeNode> | undefined = callLikeExpressionMayHaveTypeArguments(node) ? node.typeArguments : undefined;
+            const instantiated = typeArgumentNodes
+                ? createSignatureInstantiation(candidate, getTypeArgumentsFromNodes(typeArgumentNodes, typeParameters, isInJavaScriptFile(node)))
+                : inferSignatureInstantiationForOverloadFailure(node, typeParameters, candidate, args);
+            candidates[bestIndex] = instantiated;
+            return instantiated;
+        }
+
+        function getTypeArgumentsFromNodes(typeArgumentNodes: ReadonlyArray<TypeNode>, typeParameters: ReadonlyArray<TypeParameter>, isJs: boolean): ReadonlyArray<Type> {
             const typeArguments = typeArgumentNodes.map(getTypeOfNode);
             while (typeArguments.length > typeParameters.length) {
                 typeArguments.pop();
             }
             while (typeArguments.length < typeParameters.length) {
-                typeArguments.push(getConstraintOfTypeParameter(typeParameters[typeArguments.length]) || getDefaultTypeArgumentType(isInJavaScriptFile(node)));
+                typeArguments.push(getConstraintOfTypeParameter(typeParameters[typeArguments.length]) || getDefaultTypeArgumentType(isJs));
             }
-            const instantiated = createSignatureInstantiation(candidate, typeArguments);
-            candidates[bestIndex] = instantiated;
-            return instantiated;
+            return typeArguments;
+        }
+
+        function inferSignatureInstantiationForOverloadFailure(node: CallLikeExpression, typeParameters: ReadonlyArray<TypeParameter>, candidate: Signature, args: ReadonlyArray<Expression>): Signature {
+            const inferenceContext = createInferenceContext(typeParameters, candidate, /*flags*/ isInJavaScriptFile(node) ? InferenceFlags.AnyDefault : InferenceFlags.None);
+            const typeArgumentTypes = inferTypeArguments(node, candidate, args, getExcludeArgument(args), inferenceContext);
+            return createSignatureInstantiation(candidate, typeArgumentTypes);
         }
 
         function getLongestCandidateIndex(candidates: Signature[], argsCount: number): number {

tests/cases/fourslash/quickInfoCanBeTruncated.ts

@@ -519,7 +519,7 @@ verify.quickInfoIs(`type Less = "_1" | "_2" | "_3" | "_4" | "_5" | "_6" | "_7" |
 goTo.marker("3");
 verify.signatureHelp({
     marker: "3",
-    text: `f(s: "_0" | "_1" | "_2" | "_3" | "_4" | "_5" | "_6" | "_7" | "_8" | "_9" | "_10" | "_11" | "_12" | "_13" | "_14" | "_15" | "_16" | "_17" | "_18" | "_19" | "_20" | "_21" | "_22" | "_23" | "_24" | ... 474 more ... | "_499", x: never, y: string): void`
+    text: `f(s: "_499", x: "_0" | "_1" | "_2" | "_3" | "_4" | "_5" | "_6" | "_7" | "_8" | "_9" | "_10" | "_11" | "_12" | "_13" | "_14" | "_15" | "_16" | "_17" | "_18" | "_19" | "_20" | "_21" | "_22" | "_23" | "_24" | ... 473 more ... | "_498", y: string): void`
 });
 goTo.marker("4");
 verify.quickInfoIs(`type Decomposed = {

tests/cases/fourslash/signatureHelpExplicitTypeArguments.ts

@@ -8,7 +8,7 @@
 
 verify.signatureHelp(
     { marker: "1", text: "f(x: number, y: string): number" },
-    { marker: "2", text: "f(x: {}, y: {}): {}" },
+    { marker: "2", text: "f(x: boolean, y: string): boolean" },
     // too few -- fill in rest with {}
     { marker: "3", text: "f(x: number, y: {}): number" },
     // too many -- ignore extra type arguments

tests/cases/fourslash/signatureHelpInference.ts

@@ -0,0 +1,12 @@
+/// <reference path='fourslash.ts' />
+
+////declare function f<T extends string>(a: T, b: T, c: T): void;
+////f("x", /**/);
+
+verify.signatureHelp({
+    marker: "",
+    text: 'f(a: "x", b: "x", c: "x"): void',
+    parameterCount: 3,
+    parameterName: "b",
+    parameterSpan: 'b: "x"',
+});

tests/cases/fourslash/trailingCommaSignatureHelp.ts

@@ -25,6 +25,6 @@ verify.signatureHelp(
     },
     {
         marker: "b",
-        text: "f(a: {}): {}",
+        text: "f(a: 2): 2",
     },
 );