Fix type parameter constraint check in comparable relation (#1471)

Author: ahejlsberg

internal/checker/relater.go

@@ -3365,15 +3365,8 @@ func (r *Relater) structuredTypeRelatedToWorker(source *Type, target *Type, repo
 		if r.relation == r.c.comparableRelation && source.flags&TypeFlagsTypeParameter != 0 {
 			// This is a carve-out in comparability to essentially forbid comparing a type parameter with another type parameter
 			// unless one extends the other. (Remember: comparability is mostly bidirectional!)
-			constraint := r.c.getConstraintOfTypeParameter(source)
-			if constraint != nil {
-				for constraint != nil && someType(constraint, func(c *Type) bool { return c.flags&TypeFlagsTypeParameter != 0 }) {
-					result = r.isRelatedTo(constraint, target, RecursionFlagsSource, false /*reportErrors*/)
-					if result != TernaryFalse {
-						return result
-					}
-					constraint = r.c.getConstraintOfTypeParameter(constraint)
-				}
+			if constraint := r.c.getConstraintOfTypeParameter(source); constraint != nil && someType(constraint, func(c *Type) bool { return c.flags&TypeFlagsTypeParameter != 0 }) {
+				return r.isRelatedTo(constraint, target, RecursionFlagsSource, false /*reportErrors*/)
 			}
 			return TernaryFalse
 		}

testdata/baselines/reference/compiler/typeParameterComparableRelation.errors.txt

@@ -0,0 +1,16 @@
+typeParameterComparableRelation.ts(6,5): error TS2352: Conversion of type 'T' to type 'A' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
+  'A' could be instantiated with an arbitrary type which could be unrelated to 'T'.
+
+
+==== typeParameterComparableRelation.ts (1 errors) ====
+    // https://github.com/microsoft/typescript-go/issues/1462
+    
+    type StringOrT<T> = T | string
+    
+    function func<A, B, T extends StringOrT<B>>(thing: T): void {
+        thing as A; // Error
+        ~~~~~~~~~~
+!!! error TS2352: Conversion of type 'T' to type 'A' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
+!!! error TS2352:   'A' could be instantiated with an arbitrary type which could be unrelated to 'T'.
+    }
+    

testdata/baselines/reference/compiler/typeParameterComparableRelation.symbols

@@ -0,0 +1,25 @@
+//// [tests/cases/compiler/typeParameterComparableRelation.ts] ////
+
+=== typeParameterComparableRelation.ts ===
+// https://github.com/microsoft/typescript-go/issues/1462
+
+type StringOrT<T> = T | string
+>StringOrT : Symbol(StringOrT, Decl(typeParameterComparableRelation.ts, 0, 0))
+>T : Symbol(T, Decl(typeParameterComparableRelation.ts, 2, 15))
+>T : Symbol(T, Decl(typeParameterComparableRelation.ts, 2, 15))
+
+function func<A, B, T extends StringOrT<B>>(thing: T): void {
+>func : Symbol(func, Decl(typeParameterComparableRelation.ts, 2, 30))
+>A : Symbol(A, Decl(typeParameterComparableRelation.ts, 4, 14))
+>B : Symbol(B, Decl(typeParameterComparableRelation.ts, 4, 16))
+>T : Symbol(T, Decl(typeParameterComparableRelation.ts, 4, 19))
+>StringOrT : Symbol(StringOrT, Decl(typeParameterComparableRelation.ts, 0, 0))
+>B : Symbol(B, Decl(typeParameterComparableRelation.ts, 4, 16))
+>thing : Symbol(thing, Decl(typeParameterComparableRelation.ts, 4, 44))
+>T : Symbol(T, Decl(typeParameterComparableRelation.ts, 4, 19))
+
+    thing as A; // Error
+>thing : Symbol(thing, Decl(typeParameterComparableRelation.ts, 4, 44))
+>A : Symbol(A, Decl(typeParameterComparableRelation.ts, 4, 14))
+}
+

testdata/baselines/reference/compiler/typeParameterComparableRelation.types

@@ -0,0 +1,17 @@
+//// [tests/cases/compiler/typeParameterComparableRelation.ts] ////
+
+=== typeParameterComparableRelation.ts ===
+// https://github.com/microsoft/typescript-go/issues/1462
+
+type StringOrT<T> = T | string
+>StringOrT : StringOrT<T>
+
+function func<A, B, T extends StringOrT<B>>(thing: T): void {
+>func : <A, B, T extends StringOrT<B>>(thing: T) => void
+>thing : T
+
+    thing as A; // Error
+>thing as A : A
+>thing : T
+}
+

testdata/tests/cases/compiler/typeParameterComparableRelation.ts

@@ -0,0 +1,10 @@
+// @strict: true
+// @noEmit: true
+
+// https://github.com/microsoft/typescript-go/issues/1462
+
+type StringOrT<T> = T | string
+
+function func<A, B, T extends StringOrT<B>>(thing: T): void {
+    thing as A; // Error
+}