cmd/compile/internal/types2: report cause for failing const conversions

Follow-up on CL 360396.

Change-Id: Icd802baffb1fef91f8fef0070b6167a438ceda1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/360795
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/conversions.go

@@ -49,8 +49,15 @@ func (check *Checker) conversion(x *operand, T Type) {
 		// converted.
 		ok = under(T).(*TypeParam).underIs(func(u Type) bool {
 			// t is nil if there are no specific type terms
-			// TODO(gri) add a cause in case of failure
-			return u != nil && constConvertibleTo(u, nil)
+			if u == nil {
+				cause = check.sprintf("%s does not contain specific types", T)
+				return false
+			}
+			if !constConvertibleTo(u, nil) {
+				cause = check.sprintf("cannot convert %s to %s (in %s)", x, u, T)
+				return false
+			}
+			return true
 		})
 		x.mode = value // type parameters are not constants
 	case x.convertibleTo(check, T, &cause):

src/cmd/compile/internal/types2/testdata/spec/conversions.go2

@@ -27,6 +27,19 @@ func _[T ~string]() {
 	var _ T = 0 // ERROR cannot use .* as T value
 }
 
+// failing const conversions of constants to type parameters report a cause
+func _[
+	T1 any,
+	T2 interface{ m() },
+	T3 ~int | ~float64 | ~bool,
+	T4 ~int | ~string,
+]() {
+	_ = T1(0 /* ERROR cannot convert 0 .* to T1\n\tT1 does not contain specific types */ )
+	_ = T2(1 /* ERROR cannot convert 1 .* to T2\n\tT2 does not contain specific types */ )
+	_ = T3(2 /* ERROR cannot convert 2 .* to T3\n\tcannot convert 2 .* to bool \(in T3\) */ )
+	_ = T4(3.14 /* ERROR cannot convert 3.14 .* to T4\n\tcannot convert 3.14 .* to int \(in T4\) */ )
+}
+
 // "x is assignable to T"
 // - tested via assignability tests