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Port TypeScript PR #62789: Fix "never nullish" diagnostic missing expressions wrapped in parentheses #2463

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Merged
RyanCavanaugh merged 3 commits into from
Jan 14, 2026
Merged

Port TypeScript PR #62789: Fix "never nullish" diagnostic missing expressions wrapped in parentheses #2463

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fd2482a
Initial plan
Copilot Jan 9, 2026
5b14d74
Port TypeScript PR #62789: Fix "never nullish" diagnostic missing exp…
Copilot Jan 9, 2026
096b320
Merge branch 'main' into copilot/port-typescript-pull-62789
RyanCavanaugh Jan 13, 2026
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28 changes: 7 additions & 21 deletions internal/checker/checker.go
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Original file line number Diff line number Diff line change
Expand Up @@ -12542,7 +12542,6 @@ func (c *Checker) checkNullishCoalesceOperands(left *ast.Node, right *ast.Node)
}
}
c.checkNullishCoalesceOperandLeft(left)
c.checkNullishCoalesceOperandRight(right)
}

func (c *Checker) checkNullishCoalesceOperandLeft(left *ast.Node) {
Expand All @@ -12557,20 +12556,6 @@ func (c *Checker) checkNullishCoalesceOperandLeft(left *ast.Node) {
}
}

func (c *Checker) checkNullishCoalesceOperandRight(right *ast.Node) {
binaryExpression := right.Parent
if binaryExpression.Parent != nil && ast.IsBinaryExpression(binaryExpression.Parent) && binaryExpression.Parent.AsBinaryExpression().OperatorToken.Kind == ast.KindQuestionQuestionToken {
rightTarget := ast.SkipOuterExpressions(right, ast.OEKAll)
nullishSemantics := c.getSyntacticNullishnessSemantics(rightTarget)
switch nullishSemantics {
case PredicateSemanticsAlways:
c.error(rightTarget, diagnostics.This_expression_is_always_nullish)
case PredicateSemanticsNever:
c.error(rightTarget, diagnostics.This_expression_is_never_nullish)
}
}
}

func (c *Checker) getSyntacticNullishnessSemantics(node *ast.Node) PredicateSemantics {
node = ast.SkipOuterExpressions(node, ast.OEKAll)
switch node.Kind {
Expand All @@ -12586,17 +12571,18 @@ func (c *Checker) getSyntacticNullishnessSemantics(node *ast.Node) PredicateSema
return PredicateSemanticsSometimes
case ast.KindBinaryExpression:
// List of operators that can produce null/undefined:
// = ??= ?? || ||= && &&=
// || ||= && &&=
switch node.AsBinaryExpression().OperatorToken.Kind {
case ast.KindEqualsToken,
ast.KindQuestionQuestionToken,
ast.KindQuestionQuestionEqualsToken,
ast.KindBarBarToken,
case ast.KindBarBarToken,
ast.KindBarBarEqualsToken,
ast.KindAmpersandAmpersandToken,
ast.KindAmpersandAmpersandEqualsToken:
return PredicateSemanticsSometimes
case ast.KindCommaToken:
// For these operator kinds, the right operand is effectively controlling
case ast.KindCommaToken,
ast.KindEqualsToken,
ast.KindQuestionQuestionToken,
ast.KindQuestionQuestionEqualsToken:
return c.getSyntacticNullishnessSemantics(node.AsBinaryExpression().Right)
}
return PredicateSemanticsNever
Expand Down
56 changes: 28 additions & 28 deletions testdata/baselines/reference/submodule/compiler/predicateSemantics.errors.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -7,26 +7,26 @@ predicateSemantics.ts(29,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(30,13): error TS2872: This kind of expression is always truthy.
predicateSemantics.ts(31,13): error TS2872: This kind of expression is always truthy.
predicateSemantics.ts(32,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(32,21): error TS2871: This expression is always nullish.
predicateSemantics.ts(32,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(33,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(34,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(34,22): error TS2871: This expression is always nullish.
predicateSemantics.ts(36,20): error TS2871: This expression is always nullish.
predicateSemantics.ts(37,20): error TS2871: This expression is always nullish.
predicateSemantics.ts(34,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(36,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(37,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(38,21): error TS2871: This expression is always nullish.
predicateSemantics.ts(39,21): error TS2871: This expression is always nullish.
predicateSemantics.ts(40,21): error TS2871: This expression is always nullish.
predicateSemantics.ts(40,29): error TS2871: This expression is always nullish.
predicateSemantics.ts(41,21): error TS2871: This expression is always nullish.
predicateSemantics.ts(42,20): error TS2881: This expression is never nullish.
predicateSemantics.ts(43,21): error TS2881: This expression is never nullish.
predicateSemantics.ts(40,21): error TS2871: This expression is always nullish.
predicateSemantics.ts(41,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(42,13): error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
predicateSemantics.ts(43,13): error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
predicateSemantics.ts(45,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(45,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(45,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(45,21): error TS2871: This expression is always nullish.
predicateSemantics.ts(45,29): error TS2871: This expression is always nullish.
predicateSemantics.ts(46,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(46,21): error TS2881: This expression is never nullish.
predicateSemantics.ts(46,13): error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
predicateSemantics.ts(47,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(47,22): error TS2881: This expression is never nullish.
predicateSemantics.ts(47,13): error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
predicateSemantics.ts(50,8): error TS2872: This kind of expression is always truthy.
predicateSemantics.ts(51,11): error TS2872: This kind of expression is always truthy.
predicateSemantics.ts(52,8): error TS2872: This kind of expression is always truthy.
Expand Down Expand Up @@ -89,22 +89,22 @@ predicateSemantics.ts(90,1): error TS2869: Right operand of ?? is unreachable be
const p07 = null ?? null ?? null;
~~~~
!!! error TS2871: This expression is always nullish.
~~~~
~~~~~~~~~~~~
!!! error TS2871: This expression is always nullish.
const p08 = null ?? opt ?? null;
~~~~
!!! error TS2871: This expression is always nullish.
const p09 = null ?? (opt ? null : undefined) ?? null;
~~~~
!!! error TS2871: This expression is always nullish.
~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
!!! error TS2871: This expression is always nullish.

const p10 = opt ?? null ?? 1;
~~~~
~~~~~~~~~~~
!!! error TS2871: This expression is always nullish.
const p11 = opt ?? null ?? null;
~~~~
~~~~~~~~~~~
!!! error TS2871: This expression is always nullish.
const p12 = opt ?? (null ?? 1);
~~~~
Expand All @@ -115,35 +115,35 @@ predicateSemantics.ts(90,1): error TS2869: Right operand of ?? is unreachable be
const p14 = opt ?? (null ?? null ?? null);
~~~~
!!! error TS2871: This expression is always nullish.
~~~~
~~~~~~~~~~~~
!!! error TS2871: This expression is always nullish.
const p15 = opt ?? (opt ? null : undefined) ?? null;
~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
!!! error TS2871: This expression is always nullish.
const p16 = opt ?? 1 ?? 2;
~
!!! error TS2881: This expression is never nullish.
~~~~~~~~
!!! error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
const p17 = opt ?? (opt ? 1 : 2) ?? 3;
~~~~~~~~~~~
!!! error TS2881: This expression is never nullish.
~~~~~~~~~~~~~~~~~~~~
!!! error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.

const p21 = null ?? null ?? null ?? null;
~~~~
!!! error TS2871: This expression is always nullish.
~~~~
~~~~~~~~~~~~
!!! error TS2871: This expression is always nullish.
~~~~
~~~~~~~~~~~~~~~~~~~~
!!! error TS2871: This expression is always nullish.
const p22 = null ?? 1 ?? 1;
~~~~
!!! error TS2871: This expression is always nullish.
~
!!! error TS2881: This expression is never nullish.
~~~~~~~~~
!!! error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
const p23 = null ?? (opt ? 1 : 2) ?? 1;
~~~~
!!! error TS2871: This expression is always nullish.
~~~~~~~~~~~
!!! error TS2881: This expression is never nullish.
~~~~~~~~~~~~~~~~~~~~~
!!! error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.

// Outer expression tests
while ({} as any) { }
Expand Down
117 changes: 117 additions & 0 deletions testdata/baselines/reference/submodule/compiler/predicateSemantics.errors.txt.diff
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Original file line number Diff line number Diff line change
@@ -0,0 +1,117 @@
--- old.predicateSemantics.errors.txt
+++ new.predicateSemantics.errors.txt
@@= skipped -6, +6 lines =@@
predicateSemantics.ts(30,13): error TS2872: This kind of expression is always truthy.
predicateSemantics.ts(31,13): error TS2872: This kind of expression is always truthy.
predicateSemantics.ts(32,13): error TS2871: This expression is always nullish.
-predicateSemantics.ts(32,21): error TS2871: This expression is always nullish.
+predicateSemantics.ts(32,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(33,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(34,13): error TS2871: This expression is always nullish.
-predicateSemantics.ts(34,22): error TS2871: This expression is always nullish.
-predicateSemantics.ts(36,20): error TS2871: This expression is always nullish.
-predicateSemantics.ts(37,20): error TS2871: This expression is always nullish.
+predicateSemantics.ts(34,13): error TS2871: This expression is always nullish.
+predicateSemantics.ts(36,13): error TS2871: This expression is always nullish.
+predicateSemantics.ts(37,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(38,21): error TS2871: This expression is always nullish.
predicateSemantics.ts(39,21): error TS2871: This expression is always nullish.
predicateSemantics.ts(40,21): error TS2871: This expression is always nullish.
-predicateSemantics.ts(40,29): error TS2871: This expression is always nullish.
-predicateSemantics.ts(41,21): error TS2871: This expression is always nullish.
-predicateSemantics.ts(42,20): error TS2881: This expression is never nullish.
-predicateSemantics.ts(43,21): error TS2881: This expression is never nullish.
-predicateSemantics.ts(45,13): error TS2871: This expression is always nullish.
-predicateSemantics.ts(45,21): error TS2871: This expression is always nullish.
-predicateSemantics.ts(45,29): error TS2871: This expression is always nullish.
+predicateSemantics.ts(40,21): error TS2871: This expression is always nullish.
+predicateSemantics.ts(41,13): error TS2871: This expression is always nullish.
+predicateSemantics.ts(42,13): error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
+predicateSemantics.ts(43,13): error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
+predicateSemantics.ts(45,13): error TS2871: This expression is always nullish.
+predicateSemantics.ts(45,13): error TS2871: This expression is always nullish.
+predicateSemantics.ts(45,13): error TS2871: This expression is always nullish.
predicateSemantics.ts(46,13): error TS2871: This expression is always nullish.
-predicateSemantics.ts(46,21): error TS2881: This expression is never nullish.
+predicateSemantics.ts(46,13): error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
predicateSemantics.ts(47,13): error TS2871: This expression is always nullish.
-predicateSemantics.ts(47,22): error TS2881: This expression is never nullish.
+predicateSemantics.ts(47,13): error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
predicateSemantics.ts(50,8): error TS2872: This kind of expression is always truthy.
predicateSemantics.ts(51,11): error TS2872: This kind of expression is always truthy.
predicateSemantics.ts(52,8): error TS2872: This kind of expression is always truthy.
@@= skipped -82, +82 lines =@@
const p07 = null ?? null ?? null;
~~~~
!!! error TS2871: This expression is always nullish.
- ~~~~
+ ~~~~~~~~~~~~
!!! error TS2871: This expression is always nullish.
const p08 = null ?? opt ?? null;
~~~~
@@= skipped -8, +8 lines =@@
const p09 = null ?? (opt ? null : undefined) ?? null;
~~~~
!!! error TS2871: This expression is always nullish.
- ~~~~~~~~~~~~~~~~~~~~~~
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
!!! error TS2871: This expression is always nullish.

const p10 = opt ?? null ?? 1;
- ~~~~
+ ~~~~~~~~~~~
!!! error TS2871: This expression is always nullish.
const p11 = opt ?? null ?? null;
- ~~~~
+ ~~~~~~~~~~~
!!! error TS2871: This expression is always nullish.
const p12 = opt ?? (null ?? 1);
~~~~
@@= skipped -18, +18 lines =@@
const p14 = opt ?? (null ?? null ?? null);
~~~~
!!! error TS2871: This expression is always nullish.
- ~~~~
+ ~~~~~~~~~~~~
!!! error TS2871: This expression is always nullish.
const p15 = opt ?? (opt ? null : undefined) ?? null;
- ~~~~~~~~~~~~~~~~~~~~~~
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
!!! error TS2871: This expression is always nullish.
const p16 = opt ?? 1 ?? 2;
- ~
-!!! error TS2881: This expression is never nullish.
+ ~~~~~~~~
+!!! error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
const p17 = opt ?? (opt ? 1 : 2) ?? 3;
- ~~~~~~~~~~~
-!!! error TS2881: This expression is never nullish.
+ ~~~~~~~~~~~~~~~~~~~~
+!!! error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.

const p21 = null ?? null ?? null ?? null;
~~~~
!!! error TS2871: This expression is always nullish.
- ~~~~
+ ~~~~~~~~~~~~
!!! error TS2871: This expression is always nullish.
- ~~~~
+ ~~~~~~~~~~~~~~~~~~~~
!!! error TS2871: This expression is always nullish.
const p22 = null ?? 1 ?? 1;
~~~~
!!! error TS2871: This expression is always nullish.
- ~
-!!! error TS2881: This expression is never nullish.
+ ~~~~~~~~~
+!!! error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.
const p23 = null ?? (opt ? 1 : 2) ?? 1;
~~~~
!!! error TS2871: This expression is always nullish.
- ~~~~~~~~~~~
-!!! error TS2881: This expression is never nullish.
+ ~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2869: Right operand of ?? is unreachable because the left operand is never nullish.

// Outer expression tests
while ({} as any) { }
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