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>

diff --git a/src/cmd/compile/internal/types2/conversions.go b/src/cmd/compile/internal/types2/conversions.go
index 4d0ed79c38b9c4c6d3521f76d6ec6e2f0357900a..5798bacca70deb7e8428b116317e460998ef3832 100644 (file)
--- a/src/cmd/compile/internal/types2/conversions.go
+++ b/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):

diff --git a/src/cmd/compile/internal/types2/testdata/spec/conversions.go2 b/src/cmd/compile/internal/types2/testdata/spec/conversions.go2
index 942d9c0f6f0ddc3b1fad1c1381ca2a9146d5a594..fde332f34bd13e091ecffc7831da54c6db900dc8 100644 (file)
--- a/src/cmd/compile/internal/types2/testdata/spec/conversions.go2
+++ b/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