diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index fe8d32647c..04729d7b83 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -17406,7 +17406,7 @@ namespace ts {
          */
         function isKnownProperty(targetType: Type, name: __String, isComparingJsxAttributes: boolean): boolean {
             if (targetType.flags & TypeFlags.Object) {
-                const resolved = resolveStructuredTypeMembers(<ObjectType>targetType);
+                const resolved = resolveStructuredTypeMembers(targetType as ObjectType);
                 if (resolved.stringIndexInfo ||
                     resolved.numberIndexInfo && isNumericLiteralName(name) ||
                     getPropertyOfObjectType(targetType, name) ||
@@ -17416,12 +17416,16 @@ namespace ts {
                 }
             }
             else if (targetType.flags & TypeFlags.UnionOrIntersection) {
-                for (const t of (<UnionOrIntersectionType>targetType).types) {
+                for (const t of (targetType as UnionOrIntersectionType).types) {
                     if (isKnownProperty(t, name, isComparingJsxAttributes)) {
                         return true;
                     }
                 }
             }
+            else if (targetType.flags & TypeFlags.Conditional) {
+                return isKnownProperty((targetType as ConditionalType).root.trueType, name, isComparingJsxAttributes) ||
+                    isKnownProperty((targetType as ConditionalType).root.falseType, name, isComparingJsxAttributes);
+            }
             return false;
         }
 
diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index c2fd1eed63..da72ebe7a3 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -1866,6 +1866,9 @@ namespace ts {
             if (candidateName !== undefined && Math.abs(candidateName.length - nameLowerCase.length) <= maximumLengthDifference) {
                 const candidateNameLowerCase = candidateName.toLowerCase();
                 if (candidateNameLowerCase === nameLowerCase) {
+                    if (candidateName === name) {
+                        continue;
+                    }
                     return candidate;
                 }
                 if (justCheckExactMatches) {
diff --git a/tests/baselines/reference/conditionalTypesExcessProperties.errors.txt b/tests/baselines/reference/conditionalTypesExcessProperties.errors.txt
new file mode 100644
index 0000000000..a01cb8c898
--- /dev/null
+++ b/tests/baselines/reference/conditionalTypesExcessProperties.errors.txt
@@ -0,0 +1,24 @@
+tests/cases/conformance/types/conditional/conditionalTypesExcessProperties.ts(8,5): error TS2322: Type '{ test: string; arg: A; }' is not assignable to type 'Something<A>'.
+  Type '{ test: string; arg: A; }' is not assignable to type 'A extends object ? { arg: A; } : { arg?: undefined; }'.
+tests/cases/conformance/types/conditional/conditionalTypesExcessProperties.ts(9,33): error TS2322: Type 'A' is not assignable to type 'Something<A>["arr"]'.
+  Type 'object' is not assignable to type 'Something<A>["arr"]'.
+
+
+==== tests/cases/conformance/types/conditional/conditionalTypesExcessProperties.ts (2 errors) ====
+    type Something<T> = { test: string } &  (T extends object ? {
+        arg: T
+    } : {
+        arg?: undefined
+        });
+    
+    function testFunc2<A extends object>(a: A, sa: Something<A>) {
+        sa = { test: 'hi', arg: a }; // not excess (but currently still not assignable)
+        ~~
+!!! error TS2322: Type '{ test: string; arg: A; }' is not assignable to type 'Something<A>'.
+!!! error TS2322:   Type '{ test: string; arg: A; }' is not assignable to type 'A extends object ? { arg: A; } : { arg?: undefined; }'.
+        sa = { test: 'bye', arg: a, arr: a } // excess
+                                    ~~~
+!!! error TS2322: Type 'A' is not assignable to type 'Something<A>["arr"]'.
+!!! error TS2322:   Type 'object' is not assignable to type 'Something<A>["arr"]'.
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/conditionalTypesExcessProperties.js b/tests/baselines/reference/conditionalTypesExcessProperties.js
new file mode 100644
index 0000000000..2dc9a0161a
--- /dev/null
+++ b/tests/baselines/reference/conditionalTypesExcessProperties.js
@@ -0,0 +1,18 @@
+//// [conditionalTypesExcessProperties.ts]
+type Something<T> = { test: string } &  (T extends object ? {
+    arg: T
+} : {
+    arg?: undefined
+    });
+
+function testFunc2<A extends object>(a: A, sa: Something<A>) {
+    sa = { test: 'hi', arg: a }; // not excess (but currently still not assignable)
+    sa = { test: 'bye', arg: a, arr: a } // excess
+}
+
+
+//// [conditionalTypesExcessProperties.js]
+function testFunc2(a, sa) {
+    sa = { test: 'hi', arg: a }; // not excess (but currently still not assignable)
+    sa = { test: 'bye', arg: a, arr: a }; // excess
+}
diff --git a/tests/baselines/reference/conditionalTypesExcessProperties.symbols b/tests/baselines/reference/conditionalTypesExcessProperties.symbols
new file mode 100644
index 0000000000..807330df85
--- /dev/null
+++ b/tests/baselines/reference/conditionalTypesExcessProperties.symbols
@@ -0,0 +1,41 @@
+=== tests/cases/conformance/types/conditional/conditionalTypesExcessProperties.ts ===
+type Something<T> = { test: string } &  (T extends object ? {
+>Something : Symbol(Something, Decl(conditionalTypesExcessProperties.ts, 0, 0))
+>T : Symbol(T, Decl(conditionalTypesExcessProperties.ts, 0, 15))
+>test : Symbol(test, Decl(conditionalTypesExcessProperties.ts, 0, 21))
+>T : Symbol(T, Decl(conditionalTypesExcessProperties.ts, 0, 15))
+
+    arg: T
+>arg : Symbol(arg, Decl(conditionalTypesExcessProperties.ts, 0, 61))
+>T : Symbol(T, Decl(conditionalTypesExcessProperties.ts, 0, 15))
+
+} : {
+    arg?: undefined
+>arg : Symbol(arg, Decl(conditionalTypesExcessProperties.ts, 2, 5))
+
+    });
+
+function testFunc2<A extends object>(a: A, sa: Something<A>) {
+>testFunc2 : Symbol(testFunc2, Decl(conditionalTypesExcessProperties.ts, 4, 7))
+>A : Symbol(A, Decl(conditionalTypesExcessProperties.ts, 6, 19))
+>a : Symbol(a, Decl(conditionalTypesExcessProperties.ts, 6, 37))
+>A : Symbol(A, Decl(conditionalTypesExcessProperties.ts, 6, 19))
+>sa : Symbol(sa, Decl(conditionalTypesExcessProperties.ts, 6, 42))
+>Something : Symbol(Something, Decl(conditionalTypesExcessProperties.ts, 0, 0))
+>A : Symbol(A, Decl(conditionalTypesExcessProperties.ts, 6, 19))
+
+    sa = { test: 'hi', arg: a }; // not excess (but currently still not assignable)
+>sa : Symbol(sa, Decl(conditionalTypesExcessProperties.ts, 6, 42))
+>test : Symbol(test, Decl(conditionalTypesExcessProperties.ts, 7, 10))
+>arg : Symbol(arg, Decl(conditionalTypesExcessProperties.ts, 7, 22))
+>a : Symbol(a, Decl(conditionalTypesExcessProperties.ts, 6, 37))
+
+    sa = { test: 'bye', arg: a, arr: a } // excess
+>sa : Symbol(sa, Decl(conditionalTypesExcessProperties.ts, 6, 42))
+>test : Symbol(test, Decl(conditionalTypesExcessProperties.ts, 8, 10))
+>arg : Symbol(arg, Decl(conditionalTypesExcessProperties.ts, 8, 23))
+>a : Symbol(a, Decl(conditionalTypesExcessProperties.ts, 6, 37))
+>arr : Symbol(arr, Decl(conditionalTypesExcessProperties.ts, 8, 31))
+>a : Symbol(a, Decl(conditionalTypesExcessProperties.ts, 6, 37))
+}
+
diff --git a/tests/baselines/reference/conditionalTypesExcessProperties.types b/tests/baselines/reference/conditionalTypesExcessProperties.types
new file mode 100644
index 0000000000..7bbab8bfa1
--- /dev/null
+++ b/tests/baselines/reference/conditionalTypesExcessProperties.types
@@ -0,0 +1,47 @@
+=== tests/cases/conformance/types/conditional/conditionalTypesExcessProperties.ts ===
+type Something<T> = { test: string } &  (T extends object ? {
+>Something : Something<T>
+>T : T
+>test : string
+>T : T
+
+    arg: T
+>arg : T
+>T : T
+
+} : {
+    arg?: undefined
+>arg : undefined
+
+    });
+
+function testFunc2<A extends object>(a: A, sa: Something<A>) {
+>testFunc2 : <A extends object>(a: A, sa: Something<A>) => void
+>A : A
+>a : A
+>A : A
+>sa : Something<A>
+>Something : Something<T>
+>A : A
+
+    sa = { test: 'hi', arg: a }; // not excess (but currently still not assignable)
+>sa = { test: 'hi', arg: a } : { test: string; arg: A; }
+>sa : Something<A>
+>{ test: 'hi', arg: a } : { test: string; arg: A; }
+>test : string
+>'hi' : "hi"
+>arg : A
+>a : A
+
+    sa = { test: 'bye', arg: a, arr: a } // excess
+>sa = { test: 'bye', arg: a, arr: a } : { test: string; arg: A; arr: A; }
+>sa : Something<A>
+>{ test: 'bye', arg: a, arr: a } : { test: string; arg: A; arr: A; }
+>test : string
+>'bye' : "bye"
+>arg : A
+>a : A
+>arr : A
+>a : A
+}
+
diff --git a/tests/cases/conformance/types/conditional/conditionalTypesExcessProperties.ts b/tests/cases/conformance/types/conditional/conditionalTypesExcessProperties.ts
new file mode 100644
index 0000000000..136231d654
--- /dev/null
+++ b/tests/cases/conformance/types/conditional/conditionalTypesExcessProperties.ts
@@ -0,0 +1,10 @@
+type Something<T> = { test: string } &  (T extends object ? {
+    arg: T
+} : {
+    arg?: undefined
+    });
+
+function testFunc2<A extends object>(a: A, sa: Something<A>) {
+    sa = { test: 'hi', arg: a }; // not excess (but currently still not assignable)
+    sa = { test: 'bye', arg: a, arr: a } // excess
+}