go/types, types2: delay "does not satisfy comparable" error until needed

Fixes #49112.

Change-Id: I8effbca7bcbb257b18fd4d3d1914fd10d4afaaae
Reviewed-on: https://go-review.googlesource.com/c/go/+/372594
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

4 files changed, 32 insertions, 20 deletions

diff --git a/src/cmd/compile/internal/types2/instantiate.go b/src/cmd/compile/internal/types2/instantiate.go
index cda6c7baf4..b2e1087c41 100644
--- a/src/cmd/compile/internal/types2/instantiate.go
+++ b/src/cmd/compile/internal/types2/instantiate.go
@@ -192,17 +192,7 @@ func (check *Checker) implements(V, T Type, qf Qualifier) error {
 		return errorf("cannot implement %s (empty type set)", T)
 	}
 
-	// If T is comparable, V must be comparable.
-	// TODO(gri) the error messages could be better, here
-	if Ti.IsComparable() && !Comparable(V) {
-		if Vi != nil && Vi.Empty() {
-			return errorf("empty interface %s does not implement %s", V, T)
-		}
-		return errorf("%s does not implement comparable", V)
-	}
-
-	// V must implement T (methods)
-	// - check only if we have methods
+	// V must implement T's methods, if any.
 	if Ti.NumMethods() > 0 {
 		if m, wrong := check.missingMethod(V, Ti, true); m != nil {
 			// TODO(gri) needs to print updated name to avoid major confusion in error message!
@@ -220,10 +210,17 @@ func (check *Checker) implements(V, T Type, qf Qualifier) error {
 		}
 	}
 
+	// If T is comparable, V must be comparable.
+	// Remember as a pending error and report only if we don't have a more specific error.
+	var pending error
+	if Ti.IsComparable() && !Comparable(V) {
+		pending = errorf("%s does not implement comparable", V)
+	}
+
 	// V must also be in the set of types of T, if any.
 	// Constraints with empty type sets were already excluded above.
 	if !Ti.typeSet().hasTerms() {
-		return nil // nothing to do
+		return pending // nothing to do
 	}
 
 	// If V is itself an interface, each of its possible types must be in the set
@@ -234,7 +231,7 @@ func (check *Checker) implements(V, T Type, qf Qualifier) error {
 			// TODO(gri) report which type is missing
 			return errorf("%s does not implement %s", V, T)
 		}
-		return nil
+		return pending
 	}
 
 	// Otherwise, V's type must be included in the iface type set.
@@ -262,5 +259,5 @@ func (check *Checker) implements(V, T Type, qf Qualifier) error {
 		}
 	}
 
-	return nil
+	return pending
 }
diff --git a/src/cmd/compile/internal/types2/testdata/check/issues.go2 b/src/cmd/compile/internal/types2/testdata/check/issues.go2
index 76f9cc5010..5b6eebd4fd 100644
--- a/src/cmd/compile/internal/types2/testdata/check/issues.go2
+++ b/src/cmd/compile/internal/types2/testdata/check/issues.go2
@@ -58,7 +58,7 @@ func _() {
 type T1[P interface{~uint}] struct{}
 
 func _[P any]() {
-    _ = T1[P /* ERROR empty interface P does not implement interface{~uint} */ ]{}
+    _ = T1[P /* ERROR P does not implement interface{~uint} */ ]{}
 }
 
 // This is the original (simplified) program causing the same issue.
@@ -74,8 +74,8 @@ func (u T2[U]) Add1() U {
     return u.s + 1
 }
 
-func NewT2[U any]() T2[U /* ERROR empty interface U does not implement Unsigned */ ] {
-    return T2[U /* ERROR empty interface U does not implement Unsigned */ ]{}
+func NewT2[U any]() T2[U /* ERROR U does not implement Unsigned */ ] {
+    return T2[U /* ERROR U does not implement Unsigned */ ]{}
 }
 
 func _() {
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47411.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47411.go2
index ce5db0a615..3f405baed7 100644
--- a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47411.go2
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47411.go2
@@ -16,11 +16,11 @@ func _[P comparable,
         _ = f[P]
         _ = f[Q]
         _ = f[func( /* ERROR does not implement comparable */ )]
-        _ = f[R /* ERROR empty interface R does not implement comparable */ ]
+        _ = f[R /* ERROR R does not implement comparable */ ]
 
         _ = g[int]
         _ = g[P /* ERROR P does not implement interface{interface{comparable; ~int\|~string} */ ]
         _ = g[Q]
-        _ = g[func( /* ERROR does not implement comparable */ )]
-        _ = g[R /* ERROR empty interface R does not implement interface{interface{comparable; ~int\|~string} */ ]
+        _ = g[func( /* ERROR func\(\) does not implement interface{interface{comparable; ~int\|~string}} */ )]
+        _ = g[R /* ERROR R does not implement interface{interface{comparable; ~int\|~string} */ ]
 }
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue49112.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue49112.go2
new file mode 100644
index 0000000000..0efc9066ec
--- /dev/null
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue49112.go2
@@ -0,0 +1,15 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f[P int](P) {}
+
+func _() {
+        _ = f[int]
+        _ = f[[ /* ERROR \[\]int does not implement int */ ]int]
+
+        f(0)
+        f( /* ERROR \[\]int does not implement int */ []int{})
+}