Only get the apparent type of a contextual type when needed

src/compiler/checker.ts

@@ -6966,7 +6966,7 @@ namespace ts {
             else if (operator === SyntaxKind.BarBarToken) {
                 // When an || expression has a contextual type, the operands are contextually typed by that type. When an ||
                 // expression has no contextual type, the right operand is contextually typed by the type of the left operand.
-                let type = getApparentTypeOfContextualType(binaryExpression);
+                let type = getContextualType(binaryExpression);
                 if (!type && node === binaryExpression.right) {
                     type = checkExpression(binaryExpression.left);
                 }
@@ -7081,7 +7081,7 @@ namespace ts {
         // In a contextually typed conditional expression, the true/false expressions are contextually typed by the same type.
         function getContextualTypeForConditionalOperand(node: Expression): Type {
             const conditional = <ConditionalExpression>node.parent;
-            return node === conditional.whenTrue || node === conditional.whenFalse ? getApparentTypeOfContextualType(conditional) : undefined;
+            return node === conditional.whenTrue || node === conditional.whenFalse ? getContextualType(conditional) : undefined;
         }
 
         function getContextualTypeForJsxExpression(expr: JsxExpression | JsxSpreadAttribute): Type {
@@ -7114,9 +7114,16 @@ namespace ts {
         /**
          * Woah! Do you really want to use this function?
          *
-         * Unless you're trying to get the *non-apparent* type for a value-literal type,
+         * Unless you're trying to get the *non-apparent* type for a
+         * value-literal type or you're authoring relevant portions of this algorithm,
          * you probably meant to use 'getApparentTypeOfContextualType'.
-         * Otherwise this is slightly less useful.
+         * Otherwise this may not be very useful.
+         *
+         * In cases where you *are* working on this function, you should understand
+         * when it is appropriate to use 'getContextualType' and 'getApparentTypeOfContetxualType'.
+         *
+         *   - Use 'getContextualType' when you are simply going to propagate the result to the expression.
+         *   - Use 'getApparentTypeOfContextualType' when you're going to need the members of the type.
          *
          * @param node the expression whose contextual type will be returned.
          * @returns the contextual type of an expression.
@@ -7160,7 +7167,7 @@ namespace ts {
                     Debug.assert(parent.parent.kind === SyntaxKind.TemplateExpression);
                     return getContextualTypeForSubstitutionExpression(<TemplateExpression>parent.parent, node);
                 case SyntaxKind.ParenthesizedExpression:
-                    return getApparentTypeOfContextualType(<ParenthesizedExpression>parent);
+                    return getContextualType(<ParenthesizedExpression>parent);
                 case SyntaxKind.JsxExpression:
                 case SyntaxKind.JsxSpreadAttribute:
                     return getContextualTypeForJsxExpression(<JsxExpression>parent);

tests/baselines/reference/stringLiteralTypesAndLogicalOrExpressions01.js

@@ -0,0 +1,26 @@
+//// [stringLiteralTypesAndLogicalOrExpressions01.ts]
+
+declare function myRandBool(): boolean;
+
+let a: "foo" = "foo";
+let b = a || "foo";
+let c: "foo" = b;
+let d = b || "bar";
+let e: "foo" | "bar" = d;
+
+
+//// [stringLiteralTypesAndLogicalOrExpressions01.js]
+var a = "foo";
+var b = a || "foo";
+var c = b;
+var d = b || "bar";
+var e = d;
+
+
+//// [stringLiteralTypesAndLogicalOrExpressions01.d.ts]
+declare function myRandBool(): boolean;
+declare let a: "foo";
+declare let b: "foo";
+declare let c: "foo";
+declare let d: "foo" | "bar";
+declare let e: "foo" | "bar";

tests/baselines/reference/stringLiteralTypesAndLogicalOrExpressions01.symbols

@@ -0,0 +1,24 @@
+=== tests/cases/conformance/types/stringLiteral/stringLiteralTypesAndLogicalOrExpressions01.ts ===
+
+declare function myRandBool(): boolean;
+>myRandBool : Symbol(myRandBool, Decl(stringLiteralTypesAndLogicalOrExpressions01.ts, 0, 0))
+
+let a: "foo" = "foo";
+>a : Symbol(a, Decl(stringLiteralTypesAndLogicalOrExpressions01.ts, 3, 3))
+
+let b = a || "foo";
+>b : Symbol(b, Decl(stringLiteralTypesAndLogicalOrExpressions01.ts, 4, 3))
+>a : Symbol(a, Decl(stringLiteralTypesAndLogicalOrExpressions01.ts, 3, 3))
+
+let c: "foo" = b;
+>c : Symbol(c, Decl(stringLiteralTypesAndLogicalOrExpressions01.ts, 5, 3))
+>b : Symbol(b, Decl(stringLiteralTypesAndLogicalOrExpressions01.ts, 4, 3))
+
+let d = b || "bar";
+>d : Symbol(d, Decl(stringLiteralTypesAndLogicalOrExpressions01.ts, 6, 3))
+>b : Symbol(b, Decl(stringLiteralTypesAndLogicalOrExpressions01.ts, 4, 3))
+
+let e: "foo" | "bar" = d;
+>e : Symbol(e, Decl(stringLiteralTypesAndLogicalOrExpressions01.ts, 7, 3))
+>d : Symbol(d, Decl(stringLiteralTypesAndLogicalOrExpressions01.ts, 6, 3))
+

tests/baselines/reference/stringLiteralTypesAndLogicalOrExpressions01.types

@@ -0,0 +1,29 @@
+=== tests/cases/conformance/types/stringLiteral/stringLiteralTypesAndLogicalOrExpressions01.ts ===
+
+declare function myRandBool(): boolean;
+>myRandBool : () => boolean
+
+let a: "foo" = "foo";
+>a : "foo"
+>"foo" : "foo"
+
+let b = a || "foo";
+>b : "foo"
+>a || "foo" : "foo"
+>a : "foo"
+>"foo" : "foo"
+
+let c: "foo" = b;
+>c : "foo"
+>b : "foo"
+
+let d = b || "bar";
+>d : "foo" | "bar"
+>b || "bar" : "foo" | "bar"
+>b : "foo"
+>"bar" : "bar"
+
+let e: "foo" | "bar" = d;
+>e : "foo" | "bar"
+>d : "foo" | "bar"
+

tests/baselines/reference/stringLiteralTypesAndParenthesizedExpressions01.js

@@ -0,0 +1,23 @@
+//// [stringLiteralTypesAndParenthesizedExpressions01.ts]
+
+declare function myRandBool(): boolean;
+
+let a: "foo" = ("foo");
+let b: "foo" | "bar" = ("foo");
+let c: "foo" = (myRandBool ? "foo" : ("foo"));
+let d: "foo" | "bar" = (myRandBool ? "foo" : ("bar"));
+
+
+//// [stringLiteralTypesAndParenthesizedExpressions01.js]
+var a = ("foo");
+var b = ("foo");
+var c = (myRandBool ? "foo" : ("foo"));
+var d = (myRandBool ? "foo" : ("bar"));
+
+
+//// [stringLiteralTypesAndParenthesizedExpressions01.d.ts]
+declare function myRandBool(): boolean;
+declare let a: "foo";
+declare let b: "foo" | "bar";
+declare let c: "foo";
+declare let d: "foo" | "bar";

tests/baselines/reference/stringLiteralTypesAndParenthesizedExpressions01.symbols

@@ -0,0 +1,19 @@
+=== tests/cases/conformance/types/stringLiteral/stringLiteralTypesAndParenthesizedExpressions01.ts ===
+
+declare function myRandBool(): boolean;
+>myRandBool : Symbol(myRandBool, Decl(stringLiteralTypesAndParenthesizedExpressions01.ts, 0, 0))
+
+let a: "foo" = ("foo");
+>a : Symbol(a, Decl(stringLiteralTypesAndParenthesizedExpressions01.ts, 3, 3))
+
+let b: "foo" | "bar" = ("foo");
+>b : Symbol(b, Decl(stringLiteralTypesAndParenthesizedExpressions01.ts, 4, 3))
+
+let c: "foo" = (myRandBool ? "foo" : ("foo"));
+>c : Symbol(c, Decl(stringLiteralTypesAndParenthesizedExpressions01.ts, 5, 3))
+>myRandBool : Symbol(myRandBool, Decl(stringLiteralTypesAndParenthesizedExpressions01.ts, 0, 0))
+
+let d: "foo" | "bar" = (myRandBool ? "foo" : ("bar"));
+>d : Symbol(d, Decl(stringLiteralTypesAndParenthesizedExpressions01.ts, 6, 3))
+>myRandBool : Symbol(myRandBool, Decl(stringLiteralTypesAndParenthesizedExpressions01.ts, 0, 0))
+

tests/baselines/reference/stringLiteralTypesAndParenthesizedExpressions01.types

@@ -0,0 +1,33 @@
+=== tests/cases/conformance/types/stringLiteral/stringLiteralTypesAndParenthesizedExpressions01.ts ===
+
+declare function myRandBool(): boolean;
+>myRandBool : () => boolean
+
+let a: "foo" = ("foo");
+>a : "foo"
+>("foo") : "foo"
+>"foo" : "foo"
+
+let b: "foo" | "bar" = ("foo");
+>b : "foo" | "bar"
+>("foo") : "foo"
+>"foo" : "foo"
+
+let c: "foo" = (myRandBool ? "foo" : ("foo"));
+>c : "foo"
+>(myRandBool ? "foo" : ("foo")) : "foo"
+>myRandBool ? "foo" : ("foo") : "foo"
+>myRandBool : () => boolean
+>"foo" : "foo"
+>("foo") : "foo"
+>"foo" : "foo"
+
+let d: "foo" | "bar" = (myRandBool ? "foo" : ("bar"));
+>d : "foo" | "bar"
+>(myRandBool ? "foo" : ("bar")) : "foo" | "bar"
+>myRandBool ? "foo" : ("bar") : "foo" | "bar"
+>myRandBool : () => boolean
+>"foo" : "foo"
+>("bar") : "bar"
+>"bar" : "bar"
+

tests/baselines/reference/stringLiteralTypesAsTypeParameterConstraint01.js

@@ -0,0 +1,68 @@
+//// [stringLiteralTypesAsTypeParameterConstraint01.ts]
+
+function foo<T extends "foo">(f: (x: T) => T) {
+    return f;
+}
+
+function bar<T extends "foo" | "bar">(f: (x: T) => T) {
+    return f;
+}
+
+let f = foo(x => x);
+let fResult = f("foo");
+
+let g = foo((x => x));
+let gResult = g("foo");
+
+let h = bar(x => x);
+let hResult = h("foo");
+hResult = h("bar");
+
+//// [stringLiteralTypesAsTypeParameterConstraint01.js]
+function foo(f) {
+    return f;
+}
+function bar(f) {
+    return f;
+}
+var f = foo(function (x) { return x; });
+var fResult = f("foo");
+var g = foo((function (x) { return x; }));
+var gResult = g("foo");
+var h = bar(function (x) { return x; });
+var hResult = h("foo");
+hResult = h("bar");
+
+
+//// [stringLiteralTypesAsTypeParameterConstraint01.d.ts]
+declare function foo<T extends "foo">(f: (x: T) => T): (x: T) => T;
+declare function bar<T extends "foo" | "bar">(f: (x: T) => T): (x: T) => T;
+declare let f: (x: "foo") => "foo";
+declare let fResult: "foo";
+declare let g: (x: "foo") => "foo";
+declare let gResult: "foo";
+declare let h: (x: "foo" | "bar") => "foo" | "bar";
+declare let hResult: "foo" | "bar";
+
+
+//// [DtsFileErrors]
+
+
+tests/cases/conformance/types/stringLiteral/stringLiteralTypesAsTypeParameterConstraint01.d.ts(3,16): error TS2382: Specialized overload signature is not assignable to any non-specialized signature.
+tests/cases/conformance/types/stringLiteral/stringLiteralTypesAsTypeParameterConstraint01.d.ts(5,16): error TS2382: Specialized overload signature is not assignable to any non-specialized signature.
+
+
+==== tests/cases/conformance/types/stringLiteral/stringLiteralTypesAsTypeParameterConstraint01.d.ts (2 errors) ====
+    declare function foo<T extends "foo">(f: (x: T) => T): (x: T) => T;
+    declare function bar<T extends "foo" | "bar">(f: (x: T) => T): (x: T) => T;
+    declare let f: (x: "foo") => "foo";
+                   ~~~~~~~~~~~~~~~~~~~
+!!! error TS2382: Specialized overload signature is not assignable to any non-specialized signature.
+    declare let fResult: "foo";
+    declare let g: (x: "foo") => "foo";
+                   ~~~~~~~~~~~~~~~~~~~
+!!! error TS2382: Specialized overload signature is not assignable to any non-specialized signature.
+    declare let gResult: "foo";
+    declare let h: (x: "foo" | "bar") => "foo" | "bar";
+    declare let hResult: "foo" | "bar";
+

tests/baselines/reference/stringLiteralTypesAsTypeParameterConstraint01.symbols

@@ -0,0 +1,60 @@
+=== tests/cases/conformance/types/stringLiteral/stringLiteralTypesAsTypeParameterConstraint01.ts ===
+
+function foo<T extends "foo">(f: (x: T) => T) {
+>foo : Symbol(foo, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 0, 0))
+>T : Symbol(T, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 1, 13))
+>f : Symbol(f, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 1, 30))
+>x : Symbol(x, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 1, 34))
+>T : Symbol(T, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 1, 13))
+>T : Symbol(T, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 1, 13))
+
+    return f;
+>f : Symbol(f, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 1, 30))
+}
+
+function bar<T extends "foo" | "bar">(f: (x: T) => T) {
+>bar : Symbol(bar, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 3, 1))
+>T : Symbol(T, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 5, 13))
+>f : Symbol(f, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 5, 38))
+>x : Symbol(x, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 5, 42))
+>T : Symbol(T, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 5, 13))
+>T : Symbol(T, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 5, 13))
+
+    return f;
+>f : Symbol(f, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 5, 38))
+}
+
+let f = foo(x => x);
+>f : Symbol(f, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 9, 3))
+>foo : Symbol(foo, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 0, 0))
+>x : Symbol(x, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 9, 12))
+>x : Symbol(x, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 9, 12))
+
+let fResult = f("foo");
+>fResult : Symbol(fResult, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 10, 3))
+>f : Symbol(f, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 9, 3))
+
+let g = foo((x => x));
+>g : Symbol(g, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 12, 3))
+>foo : Symbol(foo, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 0, 0))
+>x : Symbol(x, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 12, 13))
+>x : Symbol(x, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 12, 13))
+
+let gResult = g("foo");
+>gResult : Symbol(gResult, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 13, 3))
+>g : Symbol(g, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 12, 3))
+
+let h = bar(x => x);
+>h : Symbol(h, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 15, 3))
+>bar : Symbol(bar, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 3, 1))
+>x : Symbol(x, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 15, 12))
+>x : Symbol(x, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 15, 12))
+
+let hResult = h("foo");
+>hResult : Symbol(hResult, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 16, 3))
+>h : Symbol(h, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 15, 3))
+
+hResult = h("bar");
+>hResult : Symbol(hResult, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 16, 3))
+>h : Symbol(h, Decl(stringLiteralTypesAsTypeParameterConstraint01.ts, 15, 3))
+