Fix crash when creating a union signature from signatures that do and don’t have this types

src/compiler/checker.ts

@@ -7052,10 +7052,10 @@ namespace ts {
                             // Union the result types when more than one signature matches
                             if (unionSignatures.length > 1) {
                                 let thisParameter = signature.thisParameter;
-                                if (forEach(unionSignatures, sig => sig.thisParameter)) {
-                                    // TODO: GH#18217 We tested that *some* has thisParameter and now act as if *all* do
+                                const firstThisParameterOfUnionSignatures = forEach(unionSignatures, sig => sig.thisParameter);
+                                if (firstThisParameterOfUnionSignatures) {
                                     const thisType = getUnionType(map(unionSignatures, sig => sig.thisParameter ? getTypeOfSymbol(sig.thisParameter) : anyType), UnionReduction.Subtype);
-                                    thisParameter = createSymbolWithType(signature.thisParameter!, thisType);
+                                    thisParameter = createSymbolWithType(firstThisParameterOfUnionSignatures, thisType);
                                 }
                                 s = createUnionSignature(signature, unionSignatures);
                                 s.thisParameter = thisParameter;

tests/baselines/reference/unionTypeCallSignatures5.js

@@ -0,0 +1,17 @@
+//// [unionTypeCallSignatures5.ts]
+// #31485
+interface A {
+  (this: void, b?: number): void;
+}
+interface B {
+  (this: number, b?: number): void;
+}
+interface C {
+  (i: number): void;
+}
+declare const fn: A | B | C;
+fn(0);
+
+
+//// [unionTypeCallSignatures5.js]
+fn(0);

tests/baselines/reference/unionTypeCallSignatures5.symbols

@@ -0,0 +1,31 @@
+=== tests/cases/conformance/types/union/unionTypeCallSignatures5.ts ===
+// #31485
+interface A {
+>A : Symbol(A, Decl(unionTypeCallSignatures5.ts, 0, 0))
+
+  (this: void, b?: number): void;
+>this : Symbol(this, Decl(unionTypeCallSignatures5.ts, 2, 3))
+>b : Symbol(b, Decl(unionTypeCallSignatures5.ts, 2, 14))
+}
+interface B {
+>B : Symbol(B, Decl(unionTypeCallSignatures5.ts, 3, 1))
+
+  (this: number, b?: number): void;
+>this : Symbol(this, Decl(unionTypeCallSignatures5.ts, 5, 3))
+>b : Symbol(b, Decl(unionTypeCallSignatures5.ts, 5, 16))
+}
+interface C {
+>C : Symbol(C, Decl(unionTypeCallSignatures5.ts, 6, 1))
+
+  (i: number): void;
+>i : Symbol(i, Decl(unionTypeCallSignatures5.ts, 8, 3))
+}
+declare const fn: A | B | C;
+>fn : Symbol(fn, Decl(unionTypeCallSignatures5.ts, 10, 13))
+>A : Symbol(A, Decl(unionTypeCallSignatures5.ts, 0, 0))
+>B : Symbol(B, Decl(unionTypeCallSignatures5.ts, 3, 1))
+>C : Symbol(C, Decl(unionTypeCallSignatures5.ts, 6, 1))
+
+fn(0);
+>fn : Symbol(fn, Decl(unionTypeCallSignatures5.ts, 10, 13))
+

tests/baselines/reference/unionTypeCallSignatures5.types

@@ -0,0 +1,24 @@
+=== tests/cases/conformance/types/union/unionTypeCallSignatures5.ts ===
+// #31485
+interface A {
+  (this: void, b?: number): void;
+>this : void
+>b : number
+}
+interface B {
+  (this: number, b?: number): void;
+>this : number
+>b : number
+}
+interface C {
+  (i: number): void;
+>i : number
+}
+declare const fn: A | B | C;
+>fn : A | B | C
+
+fn(0);
+>fn(0) : void
+>fn : A | B | C
+>0 : 0
+

tests/cases/conformance/types/union/unionTypeCallSignatures5.ts

@@ -0,0 +1,12 @@
+// #31485
+interface A {
+  (this: void, b?: number): void;
+}
+interface B {
+  (this: number, b?: number): void;
+}
+interface C {
+  (i: number): void;
+}
+declare const fn: A | B | C;
+fn(0);