From b83148fe64111b3add3d2fea9f39d17694928285 Mon Sep 17 00:00:00 2001
From: Wesley Wigham <wewigham@microsoft.com>
Date: Wed, 5 May 2021 13:33:45 -0700
Subject: [PATCH] Unwrap substitutions on conditional check types before
 comparing them (#43888)

---
 src/compiler/checker.ts                               |  2 +-
 .../curiousNestedConditionalEvaluationResult.js       |  7 +++++++
 .../curiousNestedConditionalEvaluationResult.symbols  | 11 +++++++++++
 .../curiousNestedConditionalEvaluationResult.types    | 10 ++++++++++
 .../curiousNestedConditionalEvaluationResult.ts       |  4 ++++
 5 files changed, 33 insertions(+), 1 deletion(-)
 create mode 100644 tests/baselines/reference/curiousNestedConditionalEvaluationResult.js
 create mode 100644 tests/baselines/reference/curiousNestedConditionalEvaluationResult.symbols
 create mode 100644 tests/baselines/reference/curiousNestedConditionalEvaluationResult.types
 create mode 100644 tests/cases/compiler/curiousNestedConditionalEvaluationResult.ts

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 72251594841..4bef5f6a7cc 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -15022,7 +15022,7 @@ namespace ts {
             // purposes of resolution. This means such types aren't subject to the instatiation depth limiter.
             while (true) {
                 const isUnwrapped = isTypicalNondistributiveConditional(root);
-                const checkType = instantiateType(unwrapNondistributiveConditionalTuple(root, root.checkType), mapper);
+                const checkType = instantiateType(unwrapNondistributiveConditionalTuple(root, getActualTypeVariable(root.checkType)), mapper);
                 const checkTypeInstantiable = isGenericObjectType(checkType) || isGenericIndexType(checkType);
                 const extendsType = instantiateType(unwrapNondistributiveConditionalTuple(root, root.extendsType), mapper);
                 if (checkType === wildcardType || extendsType === wildcardType) {
diff --git a/tests/baselines/reference/curiousNestedConditionalEvaluationResult.js b/tests/baselines/reference/curiousNestedConditionalEvaluationResult.js
new file mode 100644
index 00000000000..c2b09567a59
--- /dev/null
+++ b/tests/baselines/reference/curiousNestedConditionalEvaluationResult.js
@@ -0,0 +1,7 @@
+//// [curiousNestedConditionalEvaluationResult.ts]
+// regression test for #43123
+type Hmm = [0] extends [infer T, any?] ?
+    [T, [0] extends [T] ? true : false]
+    : never
+
+//// [curiousNestedConditionalEvaluationResult.js]
diff --git a/tests/baselines/reference/curiousNestedConditionalEvaluationResult.symbols b/tests/baselines/reference/curiousNestedConditionalEvaluationResult.symbols
new file mode 100644
index 00000000000..f88f243aa8d
--- /dev/null
+++ b/tests/baselines/reference/curiousNestedConditionalEvaluationResult.symbols
@@ -0,0 +1,11 @@
+=== tests/cases/compiler/curiousNestedConditionalEvaluationResult.ts ===
+// regression test for #43123
+type Hmm = [0] extends [infer T, any?] ?
+>Hmm : Symbol(Hmm, Decl(curiousNestedConditionalEvaluationResult.ts, 0, 0))
+>T : Symbol(T, Decl(curiousNestedConditionalEvaluationResult.ts, 1, 29))
+
+    [T, [0] extends [T] ? true : false]
+>T : Symbol(T, Decl(curiousNestedConditionalEvaluationResult.ts, 1, 29))
+>T : Symbol(T, Decl(curiousNestedConditionalEvaluationResult.ts, 1, 29))
+
+    : never
diff --git a/tests/baselines/reference/curiousNestedConditionalEvaluationResult.types b/tests/baselines/reference/curiousNestedConditionalEvaluationResult.types
new file mode 100644
index 00000000000..1a685371462
--- /dev/null
+++ b/tests/baselines/reference/curiousNestedConditionalEvaluationResult.types
@@ -0,0 +1,10 @@
+=== tests/cases/compiler/curiousNestedConditionalEvaluationResult.ts ===
+// regression test for #43123
+type Hmm = [0] extends [infer T, any?] ?
+>Hmm : [0, true]
+
+    [T, [0] extends [T] ? true : false]
+>true : true
+>false : false
+
+    : never
diff --git a/tests/cases/compiler/curiousNestedConditionalEvaluationResult.ts b/tests/cases/compiler/curiousNestedConditionalEvaluationResult.ts
new file mode 100644
index 00000000000..26784d46659
--- /dev/null
+++ b/tests/cases/compiler/curiousNestedConditionalEvaluationResult.ts
@@ -0,0 +1,4 @@
+// regression test for #43123
+type Hmm = [0] extends [infer T, any?] ?
+    [T, [0] extends [T] ? true : false]
+    : never
\ No newline at end of file