Expand exception to contravariant constraint elision to all type variables

Author: weswigham

src/compiler/checker.ts

@@ -12930,7 +12930,7 @@ namespace ts {
                 }
                 // Always substitute on type parameters, regardless of variance, since even
                 // in contravarrying positions, they may be reliant on subtuted constraints to be valid
-                if ((covariant || type.flags & TypeFlags.TypeParameter) && parent.kind === SyntaxKind.ConditionalType && node === (<ConditionalTypeNode>parent).trueType) {
+                if ((covariant || type.flags & TypeFlags.TypeVariable) && parent.kind === SyntaxKind.ConditionalType && node === (<ConditionalTypeNode>parent).trueType) {
                     const constraint = getImpliedConstraint(type, (<ConditionalTypeNode>parent).checkType, (<ConditionalTypeNode>parent).extendsType);
                     if (constraint) {
                         constraints = append(constraints, constraint);

tests/baselines/reference/callOfConditionalTypeWithConcreteBranches.js

@@ -32,9 +32,18 @@ fn2<number>(m => m(42));
 // webidl-conversions example where substituion must occur, despite contravariance of the position
 // due to the invariant usage in `Parameters`
 
-type X<V> = V extends (...args: any[]) => any ? (...args: Parameters<V>) => void : Function;
+type X<V> = V extends (...args: any[]) => any ? (...args: Parameters<V>) => void : Function;
+
+// vscode - another `Parameters` example
+export type AddFirstParameterToFunctions<Target> = {
+  [K in keyof Target]: Target[K] extends (...args: any[]) => void
+      ? (...args: Parameters<Target[K]>) => void
+      : void
+};
 
 //// [callOfConditionalTypeWithConcreteBranches.js]
+"use strict";
+exports.__esModule = true;
 function fn(arg) {
     // Expected: OK
     // Actual: Cannot convert 10 to number & T

tests/baselines/reference/callOfConditionalTypeWithConcreteBranches.symbols

@@ -103,3 +103,23 @@ type X<V> = V extends (...args: any[]) => any ? (...args: Parameters<V>) => void
 >V : Symbol(V, Decl(callOfConditionalTypeWithConcreteBranches.ts, 33, 7))
 >Function : Symbol(Function, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
 
+// vscode - another `Parameters` example
+export type AddFirstParameterToFunctions<Target> = {
+>AddFirstParameterToFunctions : Symbol(AddFirstParameterToFunctions, Decl(callOfConditionalTypeWithConcreteBranches.ts, 33, 92))
+>Target : Symbol(Target, Decl(callOfConditionalTypeWithConcreteBranches.ts, 36, 41))
+
+  [K in keyof Target]: Target[K] extends (...args: any[]) => void
+>K : Symbol(K, Decl(callOfConditionalTypeWithConcreteBranches.ts, 37, 3))
+>Target : Symbol(Target, Decl(callOfConditionalTypeWithConcreteBranches.ts, 36, 41))
+>Target : Symbol(Target, Decl(callOfConditionalTypeWithConcreteBranches.ts, 36, 41))
+>K : Symbol(K, Decl(callOfConditionalTypeWithConcreteBranches.ts, 37, 3))
+>args : Symbol(args, Decl(callOfConditionalTypeWithConcreteBranches.ts, 37, 42))
+
+      ? (...args: Parameters<Target[K]>) => void
+>args : Symbol(args, Decl(callOfConditionalTypeWithConcreteBranches.ts, 38, 9))
+>Parameters : Symbol(Parameters, Decl(lib.es5.d.ts, --, --))
+>Target : Symbol(Target, Decl(callOfConditionalTypeWithConcreteBranches.ts, 36, 41))
+>K : Symbol(K, Decl(callOfConditionalTypeWithConcreteBranches.ts, 37, 3))
+
+      : void
+};

tests/baselines/reference/callOfConditionalTypeWithConcreteBranches.types

@@ -97,3 +97,15 @@ type X<V> = V extends (...args: any[]) => any ? (...args: Parameters<V>) => void
 >args : any[]
 >args : Parameters<V>
 
+// vscode - another `Parameters` example
+export type AddFirstParameterToFunctions<Target> = {
+>AddFirstParameterToFunctions : AddFirstParameterToFunctions<Target>
+
+  [K in keyof Target]: Target[K] extends (...args: any[]) => void
+>args : any[]
+
+      ? (...args: Parameters<Target[K]>) => void
+>args : Parameters<Target[K]>
+
+      : void
+};

tests/cases/compiler/callOfConditionalTypeWithConcreteBranches.ts

@@ -31,4 +31,11 @@ fn2<number>(m => m(42));
 // webidl-conversions example where substituion must occur, despite contravariance of the position
 // due to the invariant usage in `Parameters`
 
-type X<V> = V extends (...args: any[]) => any ? (...args: Parameters<V>) => void : Function;
+type X<V> = V extends (...args: any[]) => any ? (...args: Parameters<V>) => void : Function;
+
+// vscode - another `Parameters` example
+export type AddFirstParameterToFunctions<Target> = {
+  [K in keyof Target]: Target[K] extends (...args: any[]) => void
+      ? (...args: Parameters<Target[K]>) => void
+      : void
+};