Fixes microsoft#32798: Narrow unkown to null|unknown type on == null guard

Co-authored-by: Hossein <[email protected]>

Author: jespino

src/compiler/checker.ts

@@ -23686,6 +23686,9 @@ namespace ts {
                     assumeTrue = !assumeTrue;
                 }
                 const valueType = getTypeOfExpression(value);
+                if ((type.flags & TypeFlags.Unknown) && (operator === SyntaxKind.EqualsEqualsToken) && (valueType.flags & TypeFlags.Null)) {
+                    return getUnionType([nullType, undefinedType]);
+                }
                 if ((type.flags & TypeFlags.Unknown) && assumeTrue && (operator === SyntaxKind.EqualsEqualsEqualsToken || operator === SyntaxKind.ExclamationEqualsEqualsToken)) {
                     if (valueType.flags & (TypeFlags.Primitive | TypeFlags.NonPrimitive)) {
                         return valueType;

tests/baselines/reference/narrowByEquality.errors.txt

@@ -1,6 +1,6 @@
-tests/cases/compiler/narrowByEquality.ts(53,15): error TS2322: Type 'string | number' is not assignable to type 'number'.
+tests/cases/compiler/narrowByEquality.ts(60,15): error TS2322: Type 'string | number' is not assignable to type 'number'.
   Type 'string' is not assignable to type 'number'.
-tests/cases/compiler/narrowByEquality.ts(54,9): error TS2322: Type 'string | number' is not assignable to type 'number'.
+tests/cases/compiler/narrowByEquality.ts(61,9): error TS2322: Type 'string | number' is not assignable to type 'number'.
   Type 'string' is not assignable to type 'number'.
 
 
@@ -53,6 +53,13 @@ tests/cases/compiler/narrowByEquality.ts(54,9): error TS2322: Type 'string | num
         xAndObj;
     }
 
+    // From issue #32798
+    declare let xUnknown: unknown
+    
+    if (xUnknown == null) {
+        xUnknown;
+    }
+    
     // Repro from #24991
 
     function test(level: number | string):number {

tests/baselines/reference/narrowByEquality.js

@@ -47,6 +47,13 @@ if (x == xAndObj) {
     xAndObj;
 }
 
+// From issue #32798
+declare let xUnknown: unknown
+
+if (xUnknown == null) {
+    xUnknown;
+}
+
 // Repro from #24991
 
 function test(level: number | string):number {
@@ -91,6 +98,9 @@ if (x == xAndObj) {
     x;
     xAndObj;
 }
+if (xUnknown == null) {
+    xUnknown;
+}
 // Repro from #24991
 function test(level) {
     if (level == +level) {

tests/baselines/reference/narrowByEquality.symbols

@@ -91,22 +91,33 @@ if (x == xAndObj) {
 >xAndObj : Symbol(xAndObj, Decl(narrowByEquality.ts, 37, 11))
 }
 
+// From issue #32798
+declare let xUnknown: unknown
+>xUnknown : Symbol(xUnknown, Decl(narrowByEquality.ts, 49, 11))
+
+if (xUnknown == null) {
+>xUnknown : Symbol(xUnknown, Decl(narrowByEquality.ts, 49, 11))
+
+    xUnknown;
+>xUnknown : Symbol(xUnknown, Decl(narrowByEquality.ts, 49, 11))
+}
+
 // Repro from #24991
 
 function test(level: number | string):number {
->test : Symbol(test, Decl(narrowByEquality.ts, 46, 1))
->level : Symbol(level, Decl(narrowByEquality.ts, 50, 14))
+>test : Symbol(test, Decl(narrowByEquality.ts, 53, 1))
+>level : Symbol(level, Decl(narrowByEquality.ts, 57, 14))
 
     if (level == +level) {
->level : Symbol(level, Decl(narrowByEquality.ts, 50, 14))
->level : Symbol(level, Decl(narrowByEquality.ts, 50, 14))
+>level : Symbol(level, Decl(narrowByEquality.ts, 57, 14))
+>level : Symbol(level, Decl(narrowByEquality.ts, 57, 14))
 
         const q2: number = level; // error
->q2 : Symbol(q2, Decl(narrowByEquality.ts, 52, 13))
->level : Symbol(level, Decl(narrowByEquality.ts, 50, 14))
+>q2 : Symbol(q2, Decl(narrowByEquality.ts, 59, 13))
+>level : Symbol(level, Decl(narrowByEquality.ts, 57, 14))
 
         return level;
->level : Symbol(level, Decl(narrowByEquality.ts, 50, 14))
+>level : Symbol(level, Decl(narrowByEquality.ts, 57, 14))
     }
     return 0;
 }

tests/baselines/reference/narrowByEquality.types

@@ -107,6 +107,19 @@ if (x == xAndObj) {
 >xAndObj : string | number | boolean
 }
 
+// From issue #32798
+declare let xUnknown: unknown
+>xUnknown : unknown
+
+if (xUnknown == null) {
+>xUnknown == null : boolean
+>xUnknown : unknown
+>null : null
+
+    xUnknown;
+>xUnknown : null | undefined
+}
+
 // Repro from #24991
 
 function test(level: number | string):number {

tests/cases/compiler/narrowByEquality.ts

@@ -48,6 +48,13 @@ if (x == xAndObj) {
     xAndObj;
 }
 
+// From issue #32798
+declare let xUnknown: unknown
+
+if (xUnknown == null) {
+    xUnknown;
+}
+
 // Repro from #24991
 
 function test(level: number | string):number {