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

@@ -68,4 +68,11 @@ tests/cases/compiler/narrowByEquality.ts(54,9): error TS2322: Type 'string | num
         }
         return 0;
     }
+    
+    // From issue #32798
+    declare let xUnknown: unknown
+    
+    if (xUnknown == null) {
+        xUnknown;
+    }
 

tests/baselines/reference/narrowByEquality.js

@@ -56,6 +56,13 @@ function test(level: number | string):number {
     }
     return 0;
 }
+
+// From issue #32798
+declare let xUnknown: unknown
+
+if (xUnknown == null) {
+    xUnknown;
+}
 
 
 //// [narrowByEquality.js]
@@ -99,3 +106,6 @@ function test(level) {
     }
     return 0;
 }
+if (xUnknown == null) {
+    xUnknown;
+}

tests/baselines/reference/narrowByEquality.symbols

@@ -111,3 +111,14 @@ function test(level: number | string):number {
     return 0;
 }
 
+// From issue #32798
+declare let xUnknown: unknown
+>xUnknown : Symbol(xUnknown, Decl(narrowByEquality.ts, 59, 11))
+
+if (xUnknown == null) {
+>xUnknown : Symbol(xUnknown, Decl(narrowByEquality.ts, 59, 11))
+
+    xUnknown;
+>xUnknown : Symbol(xUnknown, Decl(narrowByEquality.ts, 59, 11))
+}
+

tests/baselines/reference/narrowByEquality.types

@@ -130,3 +130,16 @@ function test(level: number | string):number {
 >0 : 0
 }
 
+// From issue #32798
+declare let xUnknown: unknown
+>xUnknown : unknown
+
+if (xUnknown == null) {
+>xUnknown == null : boolean
+>xUnknown : unknown
+>null : null
+
+    xUnknown;
+>xUnknown : null | undefined
+}
+

tests/cases/compiler/narrowByEquality.ts

@@ -57,3 +57,10 @@ function test(level: number | string):number {
     }
     return 0;
 }
+
+// From issue #32798
+declare let xUnknown: unknown
+
+if (xUnknown == null) {
+    xUnknown;
+}