From: Robert Findley <rfindley@google.com>
Date: Tue, 2 Nov 2021 22:42:51 +0000 (-0400)
Subject: go/types: report cause for failing const conversions
X-Git-Tag: go1.18beta1~584
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=49a00631b118834010e4d0124ccc04eeaf2cd7ac;p=gostls13.git

go/types: report cause for failing const conversions

This is a port of CL 360795 to go/types. Error messages were adjusted
accordingly, with a TODO to fix the discrepancy.

Change-Id: Ifd7d8248fa11a31fde391021f3c5f1840877892f
Reviewed-on: https://go-review.googlesource.com/c/go/+/360937
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
---

diff --git a/src/go/types/conversions.go b/src/go/types/conversions.go
index 9baad98e09..a6f0714ba0 100644
--- a/src/go/types/conversions.go
+++ b/src/go/types/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):
diff --git a/src/go/types/testdata/spec/conversions.go2 b/src/go/types/testdata/spec/conversions.go2
index 47b1f07d87..e54403cea9 100644
--- a/src/go/types/testdata/spec/conversions.go2
+++ b/src/go/types/testdata/spec/conversions.go2
@@ -27,6 +27,20 @@ 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,
+]() {
+	// TODO(rfindley): align the error formatting here with types2.
+	_ = T1(0 /* ERROR cannot convert 0 .* to T1.*T1 does not contain specific types */ )
+	_ = T2(1 /* ERROR cannot convert 1 .* to T2.*T2 does not contain specific types */ )
+	_ = T3(2 /* ERROR cannot convert 2 .* to T3.*cannot convert 2 .* to bool \(in T3\) */ )
+	_ = T4(3.14 /* ERROR cannot convert 3.14 .* to T4.*cannot convert 3.14 .* to int \(in T4\) */ )
+}
+
 // "x is assignable to T"
 // - tested via assignability tests