Remove the rule that replaces a union of literal types with the base type when comparing equality to a type that isn't a union of literal types.

src/compiler/checker.ts

@@ -22832,12 +22832,6 @@ namespace ts {
                 case SyntaxKind.ExclamationEqualsToken:
                 case SyntaxKind.EqualsEqualsEqualsToken:
                 case SyntaxKind.ExclamationEqualsEqualsToken:
-                    const leftIsLiteral = isLiteralType(leftType);
-                    const rightIsLiteral = isLiteralType(rightType);
-                    if (!leftIsLiteral || !rightIsLiteral) {
-                        leftType = leftIsLiteral ? getBaseTypeOfLiteralType(leftType) : leftType;
-                        rightType = rightIsLiteral ? getBaseTypeOfLiteralType(rightType) : rightType;
-                    }
                     if (!isTypeEqualityComparableTo(leftType, rightType) && !isTypeEqualityComparableTo(rightType, leftType)) {
                         reportOperatorError();
                     }

tests/baselines/reference/compareTypeParameterConstrainedByLiteralToLiteral.errors.txt

@@ -0,0 +1,13 @@
+tests/cases/compiler/compareTypeParameterConstrainedByLiteralToLiteral.ts(5,5): error TS2367: This condition will always return 'false' since the types 'T' and '"x"' have no overlap.
+
+
+==== tests/cases/compiler/compareTypeParameterConstrainedByLiteralToLiteral.ts (1 errors) ====
+    // Test for #26758
+
+    function foo<T extends "a" | "b">(t: T) {
+        t === "a";  // Should be allowed
+        t === "x";  // Should be error
+        ~~~~~~~~~
+!!! error TS2367: This condition will always return 'false' since the types 'T' and '"x"' have no overlap.
+    }
+

tests/baselines/reference/compareTypeParameterConstrainedByLiteralToLiteral.js

@@ -0,0 +1,15 @@
+//// [compareTypeParameterConstrainedByLiteralToLiteral.ts]
+// Test for #26758
+
+function foo<T extends "a" | "b">(t: T) {
+    t === "a";  // Should be allowed
+    t === "x";  // Should be error
+}
+
+
+//// [compareTypeParameterConstrainedByLiteralToLiteral.js]
+// Test for #26758
+function foo(t) {
+    t === "a"; // Should be allowed
+    t === "x"; // Should be error
+}

tests/baselines/reference/compareTypeParameterConstrainedByLiteralToLiteral.symbols

@@ -0,0 +1,16 @@
+=== tests/cases/compiler/compareTypeParameterConstrainedByLiteralToLiteral.ts ===
+// Test for #26758
+
+function foo<T extends "a" | "b">(t: T) {
+>foo : Symbol(foo, Decl(compareTypeParameterConstrainedByLiteralToLiteral.ts, 0, 0))
+>T : Symbol(T, Decl(compareTypeParameterConstrainedByLiteralToLiteral.ts, 2, 13))
+>t : Symbol(t, Decl(compareTypeParameterConstrainedByLiteralToLiteral.ts, 2, 34))
+>T : Symbol(T, Decl(compareTypeParameterConstrainedByLiteralToLiteral.ts, 2, 13))
+
+    t === "a";  // Should be allowed
+>t : Symbol(t, Decl(compareTypeParameterConstrainedByLiteralToLiteral.ts, 2, 34))
+
+    t === "x";  // Should be error
+>t : Symbol(t, Decl(compareTypeParameterConstrainedByLiteralToLiteral.ts, 2, 34))
+}
+

tests/baselines/reference/compareTypeParameterConstrainedByLiteralToLiteral.types

@@ -0,0 +1,18 @@
+=== tests/cases/compiler/compareTypeParameterConstrainedByLiteralToLiteral.ts ===
+// Test for #26758
+
+function foo<T extends "a" | "b">(t: T) {
+>foo : <T extends "a" | "b">(t: T) => void
+>t : T
+
+    t === "a";  // Should be allowed
+>t === "a" : boolean
+>t : T
+>"a" : "a"
+
+    t === "x";  // Should be error
+>t === "x" : boolean
+>t : T
+>"x" : "x"
+}
+

tests/baselines/reference/expr.errors.txt

@@ -1,7 +1,7 @@
 tests/cases/compiler/expr.ts(87,5): error TS2367: This condition will always return 'false' since the types 'number' and 'string' have no overlap.
-tests/cases/compiler/expr.ts(88,5): error TS2367: This condition will always return 'false' since the types 'number' and 'boolean' have no overlap.
+tests/cases/compiler/expr.ts(88,5): error TS2367: This condition will always return 'false' since the types 'number' and 'false' have no overlap.
 tests/cases/compiler/expr.ts(94,5): error TS2367: This condition will always return 'false' since the types 'string' and 'number' have no overlap.
-tests/cases/compiler/expr.ts(95,5): error TS2367: This condition will always return 'false' since the types 'string' and 'boolean' have no overlap.
+tests/cases/compiler/expr.ts(95,5): error TS2367: This condition will always return 'false' since the types 'string' and 'false' have no overlap.
 tests/cases/compiler/expr.ts(98,5): error TS2367: This condition will always return 'false' since the types 'string' and 'E' have no overlap.
 tests/cases/compiler/expr.ts(115,5): error TS2367: This condition will always return 'false' since the types 'E' and 'string' have no overlap.
 tests/cases/compiler/expr.ts(116,5): error TS2367: This condition will always return 'false' since the types 'E' and 'false' have no overlap.
@@ -161,7 +161,7 @@ tests/cases/compiler/expr.ts(242,7): error TS2363: The right-hand side of an ari
 !!! error TS2367: This condition will always return 'false' since the types 'number' and 'string' have no overlap.
         n==b;
         ~~~~
-!!! error TS2367: This condition will always return 'false' since the types 'number' and 'boolean' have no overlap.
+!!! error TS2367: This condition will always return 'false' since the types 'number' and 'false' have no overlap.
         n==i;
         n==n;
         n==e;
@@ -172,7 +172,7 @@ tests/cases/compiler/expr.ts(242,7): error TS2363: The right-hand side of an ari
 !!! error TS2367: This condition will always return 'false' since the types 'string' and 'number' have no overlap.
         s==b;
         ~~~~
-!!! error TS2367: This condition will always return 'false' since the types 'string' and 'boolean' have no overlap.
+!!! error TS2367: This condition will always return 'false' since the types 'string' and 'false' have no overlap.
         s==i;
         s==s;
         s==e;

tests/baselines/reference/for-inStatementsArrayErrors.errors.txt

@@ -1,6 +1,6 @@
 tests/cases/conformance/statements/for-inStatements/for-inStatementsArrayErrors.ts(4,16): error TS7015: Element implicitly has an 'any' type because index expression is not of type 'number'.
 tests/cases/conformance/statements/for-inStatements/for-inStatementsArrayErrors.ts(5,16): error TS2362: The left-hand side of an arithmetic operation must be of type 'any', 'number', 'bigint' or an enum type.
-tests/cases/conformance/statements/for-inStatements/for-inStatementsArrayErrors.ts(6,9): error TS2367: This condition will always return 'false' since the types 'string' and 'number' have no overlap.
+tests/cases/conformance/statements/for-inStatements/for-inStatementsArrayErrors.ts(6,9): error TS2367: This condition will always return 'false' since the types 'string' and '1' have no overlap.
 tests/cases/conformance/statements/for-inStatements/for-inStatementsArrayErrors.ts(8,16): error TS2339: Property 'unknownProperty' does not exist on type 'string'.
 tests/cases/conformance/statements/for-inStatements/for-inStatementsArrayErrors.ts(12,10): error TS2403: Subsequent variable declarations must have the same type.  Variable 'i' must be of type 'number', but here has type 'string'.
 tests/cases/conformance/statements/for-inStatements/for-inStatementsArrayErrors.ts(16,10): error TS2403: Subsequent variable declarations must have the same type.  Variable 'j' must be of type 'any', but here has type 'string'.
@@ -18,7 +18,7 @@ tests/cases/conformance/statements/for-inStatements/for-inStatementsArrayErrors.
 !!! error TS2362: The left-hand side of an arithmetic operation must be of type 'any', 'number', 'bigint' or an enum type.
         if (x === 1) {
             ~~~~~~~
-!!! error TS2367: This condition will always return 'false' since the types 'string' and 'number' have no overlap.
+!!! error TS2367: This condition will always return 'false' since the types 'string' and '1' have no overlap.
         }
         let a3 = x.unknownProperty;
                    ~~~~~~~~~~~~~~~

tests/baselines/reference/stringLiteralsAssertionsInEqualityComparisons02.errors.txt

@@ -1,5 +1,5 @@
 tests/cases/conformance/types/literal/stringLiteralsAssertionsInEqualityComparisons02.ts(3,9): error TS2367: This condition will always return 'false' since the types '"foo"' and '"baz"' have no overlap.
-tests/cases/conformance/types/literal/stringLiteralsAssertionsInEqualityComparisons02.ts(5,9): error TS2367: This condition will always return 'false' since the types 'string' and 'number' have no overlap.
+tests/cases/conformance/types/literal/stringLiteralsAssertionsInEqualityComparisons02.ts(5,9): error TS2367: This condition will always return 'false' since the types '"foo"' and 'number' have no overlap.
 tests/cases/conformance/types/literal/stringLiteralsAssertionsInEqualityComparisons02.ts(5,19): error TS2352: Conversion of type 'string' to type 'number' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 
 
@@ -12,7 +12,7 @@ tests/cases/conformance/types/literal/stringLiteralsAssertionsInEqualityComparis
     var b = "foo" !== ("bar" as "foo");
     var c = "foo" == (<number>"bar");
             ~~~~~~~~~~~~~~~~~~~~~~~~
-!!! error TS2367: This condition will always return 'false' since the types 'string' and 'number' have no overlap.
+!!! error TS2367: This condition will always return 'false' since the types '"foo"' and 'number' have no overlap.
                       ~~~~~~~~~~~~~
 !!! error TS2352: Conversion of type 'string' to type 'number' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
     var d = "foo" === ("bar" as EnhancedString);

tests/baselines/reference/stringLiteralsWithEqualityChecks03.errors.txt

@@ -1,8 +1,12 @@
 tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks03.ts(16,5): error TS2367: This condition will always return 'false' since the types '"foo"' and '"bar"' have no overlap.
+tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks03.ts(19,5): error TS2367: This condition will always return 'false' since the types 'Refrigerator | "foo"' and '"bar"' have no overlap.
+tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks03.ts(20,5): error TS2367: This condition will always return 'false' since the types '"bar"' and 'Refrigerator | "foo"' have no overlap.
 tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks03.ts(25,5): error TS2367: This condition will always return 'true' since the types '"foo"' and '"bar"' have no overlap.
+tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks03.ts(28,5): error TS2367: This condition will always return 'true' since the types 'Refrigerator | "foo"' and '"bar"' have no overlap.
+tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks03.ts(29,5): error TS2367: This condition will always return 'true' since the types '"bar"' and 'Refrigerator | "foo"' have no overlap.
 
 
-==== tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks03.ts (2 errors) ====
+==== tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks03.ts (6 errors) ====
     interface Runnable {
         isRunning: boolean;
     }
@@ -24,7 +28,11 @@ tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks03.ts(25,5
     b = "bar" === x;
     b = x === "bar";
     b = y === "bar";
+        ~~~~~~~~~~~
+!!! error TS2367: This condition will always return 'false' since the types 'Refrigerator | "foo"' and '"bar"' have no overlap.
     b = "bar" === y;
+        ~~~~~~~~~~~
+!!! error TS2367: This condition will always return 'false' since the types '"bar"' and 'Refrigerator | "foo"' have no overlap.
 
     b = x !== y;
     b = "foo" !== y
@@ -35,5 +43,9 @@ tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks03.ts(25,5
     b = "bar" !== x;
     b = x !== "bar";
     b = y !== "bar";
+        ~~~~~~~~~~~
+!!! error TS2367: This condition will always return 'true' since the types 'Refrigerator | "foo"' and '"bar"' have no overlap.
     b = "bar" !== y;
+        ~~~~~~~~~~~
+!!! error TS2367: This condition will always return 'true' since the types '"bar"' and 'Refrigerator | "foo"' have no overlap.
 

tests/baselines/reference/stringLiteralsWithEqualityChecks04.errors.txt

@@ -1,8 +1,12 @@
 tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks04.ts(16,5): error TS2367: This condition will always return 'false' since the types '"foo"' and '"bar"' have no overlap.
+tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks04.ts(19,5): error TS2367: This condition will always return 'false' since the types 'Refrigerator | "foo"' and '"bar"' have no overlap.
+tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks04.ts(20,5): error TS2367: This condition will always return 'false' since the types '"bar"' and 'Refrigerator | "foo"' have no overlap.
 tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks04.ts(25,5): error TS2367: This condition will always return 'true' since the types '"foo"' and '"bar"' have no overlap.
+tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks04.ts(28,5): error TS2367: This condition will always return 'true' since the types 'Refrigerator | "foo"' and '"bar"' have no overlap.
+tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks04.ts(29,5): error TS2367: This condition will always return 'true' since the types '"bar"' and 'Refrigerator | "foo"' have no overlap.
 
 
-==== tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks04.ts (2 errors) ====
+==== tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks04.ts (6 errors) ====
     interface Runnable {
         isRunning: boolean;
     }
@@ -24,7 +28,11 @@ tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks04.ts(25,5
     b = "bar" == x;
     b = x == "bar";
     b = y == "bar";
+        ~~~~~~~~~~
+!!! error TS2367: This condition will always return 'false' since the types 'Refrigerator | "foo"' and '"bar"' have no overlap.
     b = "bar" == y;
+        ~~~~~~~~~~
+!!! error TS2367: This condition will always return 'false' since the types '"bar"' and 'Refrigerator | "foo"' have no overlap.
 
     b = x != y;
     b = "foo" != y
@@ -35,5 +43,9 @@ tests/cases/conformance/types/literal/stringLiteralsWithEqualityChecks04.ts(25,5
     b = "bar" != x;
     b = x != "bar";
     b = y != "bar";
+        ~~~~~~~~~~
+!!! error TS2367: This condition will always return 'true' since the types 'Refrigerator | "foo"' and '"bar"' have no overlap.
     b = "bar" != y;
+        ~~~~~~~~~~
+!!! error TS2367: This condition will always return 'true' since the types '"bar"' and 'Refrigerator | "foo"' have no overlap.
 

tests/baselines/reference/symbolType9.errors.txt

@@ -1,24 +1,24 @@
-tests/cases/conformance/es6/Symbols/symbolType9.ts(3,1): error TS2367: This condition will always return 'false' since the types 'symbol' and 'boolean' have no overlap.
-tests/cases/conformance/es6/Symbols/symbolType9.ts(5,1): error TS2367: This condition will always return 'true' since the types 'number' and 'symbol' have no overlap.
-tests/cases/conformance/es6/Symbols/symbolType9.ts(7,1): error TS2367: This condition will always return 'false' since the types 'symbol' and 'number' have no overlap.
-tests/cases/conformance/es6/Symbols/symbolType9.ts(9,1): error TS2367: This condition will always return 'true' since the types 'boolean' and 'symbol' have no overlap.
+tests/cases/conformance/es6/Symbols/symbolType9.ts(3,1): error TS2367: This condition will always return 'false' since the types 'symbol' and 'true' have no overlap.
+tests/cases/conformance/es6/Symbols/symbolType9.ts(5,1): error TS2367: This condition will always return 'true' since the types '0' and 'symbol' have no overlap.
+tests/cases/conformance/es6/Symbols/symbolType9.ts(7,1): error TS2367: This condition will always return 'false' since the types 'symbol' and '1' have no overlap.
+tests/cases/conformance/es6/Symbols/symbolType9.ts(9,1): error TS2367: This condition will always return 'true' since the types 'false' and 'symbol' have no overlap.
 
 
 ==== tests/cases/conformance/es6/Symbols/symbolType9.ts (4 errors) ====
     var s = Symbol.for("equal");
     s == s;
     s == true;
     ~~~~~~~~~
-!!! error TS2367: This condition will always return 'false' since the types 'symbol' and 'boolean' have no overlap.
+!!! error TS2367: This condition will always return 'false' since the types 'symbol' and 'true' have no overlap.
     s != s;
     0 != s;
     ~~~~~~
-!!! error TS2367: This condition will always return 'true' since the types 'number' and 'symbol' have no overlap.
+!!! error TS2367: This condition will always return 'true' since the types '0' and 'symbol' have no overlap.
     s === s;
     s === 1;
     ~~~~~~~
-!!! error TS2367: This condition will always return 'false' since the types 'symbol' and 'number' have no overlap.
+!!! error TS2367: This condition will always return 'false' since the types 'symbol' and '1' have no overlap.
     s !== s;
     false !== s;
     ~~~~~~~~~~~
-!!! error TS2367: This condition will always return 'true' since the types 'boolean' and 'symbol' have no overlap.
+!!! error TS2367: This condition will always return 'true' since the types 'false' and 'symbol' have no overlap.

tests/cases/compiler/compareTypeParameterConstrainedByLiteralToLiteral.ts

@@ -0,0 +1,6 @@
+// Test for #26758
+
+function foo<T extends "a" | "b">(t: T) {
+    t === "a";  // Should be allowed
+    t === "x";  // Should be error
+}