Fix of #58996 (#59155)

Co-authored-by: Jake Bailey <5341706+jakebailey@users.noreply.github.com>

Author: MichalMarsalek

src/compiler/checker.ts

@@ -38754,7 +38754,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         // "x is T" means that x is T if and only if it returns true. If it returns false then x is not T.
         // This means that if the function is called with an argument of type trueType, there can't be anything left in the `else` branch. It must reduce to `never`.
         const falseCondition = createFlowNode(FlowFlags.FalseCondition, expr, antecedent);
-        const falseSubtype = getFlowTypeOfReference(param.name, initType, trueType, func, falseCondition);
+        const falseSubtype = getReducedType(getFlowTypeOfReference(param.name, initType, trueType, func, falseCondition));
         return falseSubtype.flags & TypeFlags.Never ? trueType : undefined;
     }
 

tests/baselines/reference/inferTypePredicates.errors.txt

@@ -343,4 +343,27 @@ inferTypePredicates.ts(205,7): error TS2741: Property 'z' is missing in type 'C1
       const rv = x === "";
       return rv satisfies boolean;
     }
+    
+    // https://github.com/microsoft/TypeScript/issues/58996
+    type Animal = {
+      breath: true,
+    };
+    
+    type Rock = {
+      breath: false,
+    };
+    
+    type Something = Animal | Rock;
+    
+    function isAnimal(something: Something): something is Animal {
+      return something.breath
+    }
+    
+    function positive(t: Something) {
+      return isAnimal(t)
+    }
+    
+    function negative(t: Something) { 
+      return !isAnimal(t)
+    }
 

tests/baselines/reference/inferTypePredicates.js

@@ -289,6 +289,29 @@ function isEmptyString(x: unknown) {
   const rv = x === "";
   return rv satisfies boolean;
 }
+
+// https://github.com/microsoft/TypeScript/issues/58996
+type Animal = {
+  breath: true,
+};
+
+type Rock = {
+  breath: false,
+};
+
+type Something = Animal | Rock;
+
+function isAnimal(something: Something): something is Animal {
+  return something.breath
+}
+
+function positive(t: Something) {
+  return isAnimal(t)
+}
+
+function negative(t: Something) { 
+  return !isAnimal(t)
+}
 
 
 //// [inferTypePredicates.js]
@@ -554,6 +577,15 @@ function isEmptyString(x) {
     var rv = x === "";
     return rv;
 }
+function isAnimal(something) {
+    return something.breath;
+}
+function positive(t) {
+    return isAnimal(t);
+}
+function negative(t) {
+    return !isAnimal(t);
+}
 
 
 //// [inferTypePredicates.d.ts]
@@ -648,3 +680,13 @@ declare const foobarPred: (fb: typeof foobar) => fb is {
 };
 declare const arrTest: Array<number>;
 declare function isEmptyString(x: unknown): x is "";
+type Animal = {
+    breath: true;
+};
+type Rock = {
+    breath: false;
+};
+type Something = Animal | Rock;
+declare function isAnimal(something: Something): something is Animal;
+declare function positive(t: Something): t is Animal;
+declare function negative(t: Something): t is Rock;

tests/baselines/reference/inferTypePredicates.symbols

@@ -802,3 +802,58 @@ function isEmptyString(x: unknown) {
 >rv : Symbol(rv, Decl(inferTypePredicates.ts, 285, 7))
 }
 
+// https://github.com/microsoft/TypeScript/issues/58996
+type Animal = {
+>Animal : Symbol(Animal, Decl(inferTypePredicates.ts, 287, 1))
+
+  breath: true,
+>breath : Symbol(breath, Decl(inferTypePredicates.ts, 290, 15))
+
+};
+
+type Rock = {
+>Rock : Symbol(Rock, Decl(inferTypePredicates.ts, 292, 2))
+
+  breath: false,
+>breath : Symbol(breath, Decl(inferTypePredicates.ts, 294, 13))
+
+};
+
+type Something = Animal | Rock;
+>Something : Symbol(Something, Decl(inferTypePredicates.ts, 296, 2))
+>Animal : Symbol(Animal, Decl(inferTypePredicates.ts, 287, 1))
+>Rock : Symbol(Rock, Decl(inferTypePredicates.ts, 292, 2))
+
+function isAnimal(something: Something): something is Animal {
+>isAnimal : Symbol(isAnimal, Decl(inferTypePredicates.ts, 298, 31))
+>something : Symbol(something, Decl(inferTypePredicates.ts, 300, 18))
+>Something : Symbol(Something, Decl(inferTypePredicates.ts, 296, 2))
+>something : Symbol(something, Decl(inferTypePredicates.ts, 300, 18))
+>Animal : Symbol(Animal, Decl(inferTypePredicates.ts, 287, 1))
+
+  return something.breath
+>something.breath : Symbol(breath, Decl(inferTypePredicates.ts, 290, 15), Decl(inferTypePredicates.ts, 294, 13))
+>something : Symbol(something, Decl(inferTypePredicates.ts, 300, 18))
+>breath : Symbol(breath, Decl(inferTypePredicates.ts, 290, 15), Decl(inferTypePredicates.ts, 294, 13))
+}
+
+function positive(t: Something) {
+>positive : Symbol(positive, Decl(inferTypePredicates.ts, 302, 1))
+>t : Symbol(t, Decl(inferTypePredicates.ts, 304, 18))
+>Something : Symbol(Something, Decl(inferTypePredicates.ts, 296, 2))
+
+  return isAnimal(t)
+>isAnimal : Symbol(isAnimal, Decl(inferTypePredicates.ts, 298, 31))
+>t : Symbol(t, Decl(inferTypePredicates.ts, 304, 18))
+}
+
+function negative(t: Something) { 
+>negative : Symbol(negative, Decl(inferTypePredicates.ts, 306, 1))
+>t : Symbol(t, Decl(inferTypePredicates.ts, 308, 18))
+>Something : Symbol(Something, Decl(inferTypePredicates.ts, 296, 2))
+
+  return !isAnimal(t)
+>isAnimal : Symbol(isAnimal, Decl(inferTypePredicates.ts, 298, 31))
+>t : Symbol(t, Decl(inferTypePredicates.ts, 308, 18))
+}
+

tests/baselines/reference/inferTypePredicates.types

@@ -1707,3 +1707,79 @@ function isEmptyString(x: unknown) {
 >   : ^^^^^^^
 }
 
+// https://github.com/microsoft/TypeScript/issues/58996
+type Animal = {
+>Animal : Animal
+>       : ^^^^^^
+
+  breath: true,
+>breath : true
+>       : ^^^^
+>true : true
+>     : ^^^^
+
+};
+
+type Rock = {
+>Rock : Rock
+>     : ^^^^
+
+  breath: false,
+>breath : false
+>       : ^^^^^
+>false : false
+>      : ^^^^^
+
+};
+
+type Something = Animal | Rock;
+>Something : Something
+>          : ^^^^^^^^^
+
+function isAnimal(something: Something): something is Animal {
+>isAnimal : (something: Something) => something is Animal
+>         : ^         ^^         ^^^^^                   
+>something : Something
+>          : ^^^^^^^^^
+
+  return something.breath
+>something.breath : boolean
+>                 : ^^^^^^^
+>something : Something
+>          : ^^^^^^^^^
+>breath : boolean
+>       : ^^^^^^^
+}
+
+function positive(t: Something) {
+>positive : (t: Something) => t is Animal
+>         : ^ ^^         ^^^^^^^^^^^^^^^^
+>t : Something
+>  : ^^^^^^^^^
+
+  return isAnimal(t)
+>isAnimal(t) : boolean
+>            : ^^^^^^^
+>isAnimal : (something: Something) => something is Animal
+>         : ^         ^^         ^^^^^                   
+>t : Something
+>  : ^^^^^^^^^
+}
+
+function negative(t: Something) { 
+>negative : (t: Something) => t is Rock
+>         : ^ ^^         ^^^^^^^^^^^^^^
+>t : Something
+>  : ^^^^^^^^^
+
+  return !isAnimal(t)
+>!isAnimal(t) : boolean
+>             : ^^^^^^^
+>isAnimal(t) : boolean
+>            : ^^^^^^^
+>isAnimal : (something: Something) => something is Animal
+>         : ^         ^^         ^^^^^                   
+>t : Something
+>  : ^^^^^^^^^
+}
+

tests/cases/compiler/inferTypePredicates.ts

@@ -289,3 +289,26 @@ function isEmptyString(x: unknown) {
   const rv = x === "";
   return rv satisfies boolean;
 }
+
+// https://github.com/microsoft/TypeScript/issues/58996
+type Animal = {
+  breath: true,
+};
+
+type Rock = {
+  breath: false,
+};
+
+type Something = Animal | Rock;
+
+function isAnimal(something: Something): something is Animal {
+  return something.breath
+}
+
+function positive(t: Something) {
+  return isAnimal(t)
+}
+
+function negative(t: Something) { 
+  return !isAnimal(t)
+}