diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 6c0e83eb339..a62e86b1ab8 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -13573,7 +13573,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
     }
 
     function getConstraintFromIndexedAccess(type: IndexedAccessType) {
-        if (isMappedTypeGenericIndexedAccess(type)) {
+        if (isMappedTypeGenericIndexedAccess(type) || isGenericMappedType(type.objectType)) {
             // For indexed access types of the form { [P in K]: E }[X], where K is non-generic and X is generic,
             // we substitute an instantiation of E where P is replaced with X.
             return substituteIndexedMappedType(type.objectType as MappedType, type.indexType);
diff --git a/tests/baselines/reference/mappedTypeConstraints2.errors.txt b/tests/baselines/reference/mappedTypeConstraints2.errors.txt
index d2bb04de624..4c64b13436f 100644
--- a/tests/baselines/reference/mappedTypeConstraints2.errors.txt
+++ b/tests/baselines/reference/mappedTypeConstraints2.errors.txt
@@ -1,10 +1,25 @@
 tests/cases/conformance/types/mapped/mappedTypeConstraints2.ts(10,11): error TS2322: Type 'Mapped2<K>[`get${K}`]' is not assignable to type '{ a: K; }'.
-  Type 'Mapped2<K>[`get${string}`]' is not assignable to type '{ a: K; }'.
+  Type '{ a: `get${K}`; }' is not assignable to type '{ a: K; }'.
+    Types of property 'a' are incompatible.
+      Type '`get${K}`' is not assignable to type 'K'.
+        '`get${K}`' is assignable to the constraint of type 'K', but 'K' could be instantiated with a different subtype of constraint 'string'.
+          Type '`get${string}`' is not assignable to type 'K'.
+            '`get${string}`' is assignable to the constraint of type 'K', but 'K' could be instantiated with a different subtype of constraint 'string'.
 tests/cases/conformance/types/mapped/mappedTypeConstraints2.ts(16,11): error TS2322: Type 'Mapped3<K>[Uppercase<K>]' is not assignable to type '{ a: K; }'.
-  Type 'Mapped3<K>[Uppercase<string>]' is not assignable to type '{ a: K; }'.
-    Type 'Mapped3<K>[string]' is not assignable to type '{ a: K; }'.
+  Type '{ a: Uppercase<K>; }' is not assignable to type '{ a: K; }'.
+    Types of property 'a' are incompatible.
+      Type 'Uppercase<K>' is not assignable to type 'K'.
+        'Uppercase<K>' is assignable to the constraint of type 'K', but 'K' could be instantiated with a different subtype of constraint 'string'.
+          Type 'Uppercase<string>' is not assignable to type 'K'.
+            'Uppercase<string>' is assignable to the constraint of type 'K', but 'K' could be instantiated with a different subtype of constraint 'string'.
+              Type 'string' is not assignable to type 'K'.
+                'string' is assignable to the constraint of type 'K', but 'K' could be instantiated with a different subtype of constraint 'string'.
 tests/cases/conformance/types/mapped/mappedTypeConstraints2.ts(25,57): error TS2322: Type 'Foo<T>[`get${T}`]' is not assignable to type 'T'.
-  'T' could be instantiated with an arbitrary type which could be unrelated to 'Foo<T>[`get${T}`]'.
+  'Foo<T>[`get${T}`]' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'string'.
+    Type '`get${T}`' is not assignable to type 'T'.
+      '`get${T}`' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'string'.
+        Type '`get${string}`' is not assignable to type 'T'.
+          '`get${string}`' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'string'.
 
 
 ==== tests/cases/conformance/types/mapped/mappedTypeConstraints2.ts (3 errors) ====
@@ -20,7 +35,12 @@ tests/cases/conformance/types/mapped/mappedTypeConstraints2.ts(25,57): error TS2
         const x: { a: K } = obj[key];  // Error
               ~
 !!! error TS2322: Type 'Mapped2<K>[`get${K}`]' is not assignable to type '{ a: K; }'.
-!!! error TS2322:   Type 'Mapped2<K>[`get${string}`]' is not assignable to type '{ a: K; }'.
+!!! error TS2322:   Type '{ a: `get${K}`; }' is not assignable to type '{ a: K; }'.
+!!! error TS2322:     Types of property 'a' are incompatible.
+!!! error TS2322:       Type '`get${K}`' is not assignable to type 'K'.
+!!! error TS2322:         '`get${K}`' is assignable to the constraint of type 'K', but 'K' could be instantiated with a different subtype of constraint 'string'.
+!!! error TS2322:           Type '`get${string}`' is not assignable to type 'K'.
+!!! error TS2322:             '`get${string}`' is assignable to the constraint of type 'K', but 'K' could be instantiated with a different subtype of constraint 'string'.
     }
     
     type Mapped3<K extends string> = { [P in K as Uppercase<P>]: { a: P } };
@@ -29,8 +49,14 @@ tests/cases/conformance/types/mapped/mappedTypeConstraints2.ts(25,57): error TS2
         const x: { a: K } = obj[key];  // Error
               ~
 !!! error TS2322: Type 'Mapped3<K>[Uppercase<K>]' is not assignable to type '{ a: K; }'.
-!!! error TS2322:   Type 'Mapped3<K>[Uppercase<string>]' is not assignable to type '{ a: K; }'.
-!!! error TS2322:     Type 'Mapped3<K>[string]' is not assignable to type '{ a: K; }'.
+!!! error TS2322:   Type '{ a: Uppercase<K>; }' is not assignable to type '{ a: K; }'.
+!!! error TS2322:     Types of property 'a' are incompatible.
+!!! error TS2322:       Type 'Uppercase<K>' is not assignable to type 'K'.
+!!! error TS2322:         'Uppercase<K>' is assignable to the constraint of type 'K', but 'K' could be instantiated with a different subtype of constraint 'string'.
+!!! error TS2322:           Type 'Uppercase<string>' is not assignable to type 'K'.
+!!! error TS2322:             'Uppercase<string>' is assignable to the constraint of type 'K', but 'K' could be instantiated with a different subtype of constraint 'string'.
+!!! error TS2322:               Type 'string' is not assignable to type 'K'.
+!!! error TS2322:                 'string' is assignable to the constraint of type 'K', but 'K' could be instantiated with a different subtype of constraint 'string'.
     }
     
     // Repro from #47794
@@ -42,7 +68,11 @@ tests/cases/conformance/types/mapped/mappedTypeConstraints2.ts(25,57): error TS2
     const get = <T extends string>(t: T, foo: Foo<T>): T => foo[`get${t}`];  // Type 'Foo<T>[`get${T}`]' is not assignable to type 'T'
                                                             ~~~~~~~~~~~~~~
 !!! error TS2322: Type 'Foo<T>[`get${T}`]' is not assignable to type 'T'.
-!!! error TS2322:   'T' could be instantiated with an arbitrary type which could be unrelated to 'Foo<T>[`get${T}`]'.
+!!! error TS2322:   'Foo<T>[`get${T}`]' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'string'.
+!!! error TS2322:     Type '`get${T}`' is not assignable to type 'T'.
+!!! error TS2322:       '`get${T}`' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'string'.
+!!! error TS2322:         Type '`get${string}`' is not assignable to type 'T'.
+!!! error TS2322:           '`get${string}`' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'string'.
     
     // Repro from #48626
     
@@ -65,4 +95,14 @@ tests/cases/conformance/types/mapped/mappedTypeConstraints2.ts(25,57): error TS2
         }
         return true;
     }
+    
+    // repro from #50030
+    
+    type ObjectWithUnderscoredKeys<K extends string> = {
+        [k in K as `_${k}`]: true;
+    };
+    
+    function genericTest<K extends string>(objectWithUnderscoredKeys: ObjectWithUnderscoredKeys<K>, key: K) {
+      const shouldBeTrue: true = objectWithUnderscoredKeys[`_${key}`];
+    }
     
\ No newline at end of file
diff --git a/tests/baselines/reference/mappedTypeConstraints2.js b/tests/baselines/reference/mappedTypeConstraints2.js
index 442865e273a..33506284285 100644
--- a/tests/baselines/reference/mappedTypeConstraints2.js
+++ b/tests/baselines/reference/mappedTypeConstraints2.js
@@ -46,6 +46,16 @@ function validate<T extends object>(obj: T, bounds: NumericBoundsOf<T>) {
     }
     return true;
 }
+
+// repro from #50030
+
+type ObjectWithUnderscoredKeys<K extends string> = {
+    [k in K as `_${k}`]: true;
+};
+
+function genericTest<K extends string>(objectWithUnderscoredKeys: ObjectWithUnderscoredKeys<K>, key: K) {
+  const shouldBeTrue: true = objectWithUnderscoredKeys[`_${key}`];
+}
 
 
 //// [mappedTypeConstraints2.js]
@@ -71,6 +81,9 @@ function validate(obj, bounds) {
     }
     return true;
 }
+function genericTest(objectWithUnderscoredKeys, key) {
+    const shouldBeTrue = objectWithUnderscoredKeys[`_${key}`];
+}
 
 
 //// [mappedTypeConstraints2.d.ts]
@@ -104,3 +117,7 @@ type NumericBoundsOf<T> = {
     [K in keyof T as T[K] extends number | undefined ? K : never]: Bounds;
 };
 declare function validate<T extends object>(obj: T, bounds: NumericBoundsOf<T>): boolean;
+type ObjectWithUnderscoredKeys<K extends string> = {
+    [k in K as `_${k}`]: true;
+};
+declare function genericTest<K extends string>(objectWithUnderscoredKeys: ObjectWithUnderscoredKeys<K>, key: K): void;
diff --git a/tests/baselines/reference/mappedTypeConstraints2.symbols b/tests/baselines/reference/mappedTypeConstraints2.symbols
index 90ee329dada..d66a412d565 100644
--- a/tests/baselines/reference/mappedTypeConstraints2.symbols
+++ b/tests/baselines/reference/mappedTypeConstraints2.symbols
@@ -171,3 +171,31 @@ function validate<T extends object>(obj: T, bounds: NumericBoundsOf<T>) {
     return true;
 }
 
+// repro from #50030
+
+type ObjectWithUnderscoredKeys<K extends string> = {
+>ObjectWithUnderscoredKeys : Symbol(ObjectWithUnderscoredKeys, Decl(mappedTypeConstraints2.ts, 46, 1))
+>K : Symbol(K, Decl(mappedTypeConstraints2.ts, 50, 31))
+
+    [k in K as `_${k}`]: true;
+>k : Symbol(k, Decl(mappedTypeConstraints2.ts, 51, 5))
+>K : Symbol(K, Decl(mappedTypeConstraints2.ts, 50, 31))
+>k : Symbol(k, Decl(mappedTypeConstraints2.ts, 51, 5))
+
+};
+
+function genericTest<K extends string>(objectWithUnderscoredKeys: ObjectWithUnderscoredKeys<K>, key: K) {
+>genericTest : Symbol(genericTest, Decl(mappedTypeConstraints2.ts, 52, 2))
+>K : Symbol(K, Decl(mappedTypeConstraints2.ts, 54, 21))
+>objectWithUnderscoredKeys : Symbol(objectWithUnderscoredKeys, Decl(mappedTypeConstraints2.ts, 54, 39))
+>ObjectWithUnderscoredKeys : Symbol(ObjectWithUnderscoredKeys, Decl(mappedTypeConstraints2.ts, 46, 1))
+>K : Symbol(K, Decl(mappedTypeConstraints2.ts, 54, 21))
+>key : Symbol(key, Decl(mappedTypeConstraints2.ts, 54, 95))
+>K : Symbol(K, Decl(mappedTypeConstraints2.ts, 54, 21))
+
+  const shouldBeTrue: true = objectWithUnderscoredKeys[`_${key}`];
+>shouldBeTrue : Symbol(shouldBeTrue, Decl(mappedTypeConstraints2.ts, 55, 7))
+>objectWithUnderscoredKeys : Symbol(objectWithUnderscoredKeys, Decl(mappedTypeConstraints2.ts, 54, 39))
+>key : Symbol(key, Decl(mappedTypeConstraints2.ts, 54, 95))
+}
+
diff --git a/tests/baselines/reference/mappedTypeConstraints2.types b/tests/baselines/reference/mappedTypeConstraints2.types
index 534e02c8518..8691626d5e5 100644
--- a/tests/baselines/reference/mappedTypeConstraints2.types
+++ b/tests/baselines/reference/mappedTypeConstraints2.types
@@ -128,3 +128,27 @@ function validate<T extends object>(obj: T, bounds: NumericBoundsOf<T>) {
 >true : true
 }
 
+// repro from #50030
+
+type ObjectWithUnderscoredKeys<K extends string> = {
+>ObjectWithUnderscoredKeys : ObjectWithUnderscoredKeys<K>
+
+    [k in K as `_${k}`]: true;
+>true : true
+
+};
+
+function genericTest<K extends string>(objectWithUnderscoredKeys: ObjectWithUnderscoredKeys<K>, key: K) {
+>genericTest : <K extends string>(objectWithUnderscoredKeys: ObjectWithUnderscoredKeys<K>, key: K) => void
+>objectWithUnderscoredKeys : ObjectWithUnderscoredKeys<K>
+>key : K
+
+  const shouldBeTrue: true = objectWithUnderscoredKeys[`_${key}`];
+>shouldBeTrue : true
+>true : true
+>objectWithUnderscoredKeys[`_${key}`] : ObjectWithUnderscoredKeys<K>[`_${K}`]
+>objectWithUnderscoredKeys : ObjectWithUnderscoredKeys<K>
+>`_${key}` : `_${K}`
+>key : K
+}
+
diff --git a/tests/cases/conformance/types/mapped/mappedTypeConstraints2.ts b/tests/cases/conformance/types/mapped/mappedTypeConstraints2.ts
index d0f1703b113..b96957b7cbd 100644
--- a/tests/cases/conformance/types/mapped/mappedTypeConstraints2.ts
+++ b/tests/cases/conformance/types/mapped/mappedTypeConstraints2.ts
@@ -49,3 +49,13 @@ function validate<T extends object>(obj: T, bounds: NumericBoundsOf<T>) {
     }
     return true;
 }
+
+// repro from #50030
+
+type ObjectWithUnderscoredKeys<K extends string> = {
+    [k in K as `_${k}`]: true;
+};
+
+function genericTest<K extends string>(objectWithUnderscoredKeys: ObjectWithUnderscoredKeys<K>, key: K) {
+  const shouldBeTrue: true = objectWithUnderscoredKeys[`_${key}`];
+}