cmd/compile/internal/types2: more consistently print "check go.mod" if language version < 1.18

If you attempt to instantiate a generic type or func and run 'go build'
with a language version < 1.18 in the 'go' directive inside the go.mod
file, cmd/compile emits a friendly message that includes the suggestion
to 'check go.mod':

    type instantiation requires go1.18 or later (-lang was set to go1.17; check go.mod)

However, if the code instead only declares a generic type or func
without instantiating, cmd/compile currently emits a less friendly
message:

    type parameters require go1.18 or later

With this CL, the error in that situation becomes:

    type parameter requires go1.18 or later (-lang was set to go1.17; check go.mod)

Within cmd/compile/internal/types2, it already calls check.versionErrorf
in a dozen or so places, including three existing calls to
check.versionErrorf within typeset.go (e.g., for embedding a constraint
interface).

This CL adds two more calls to check.versionErrorf, replacing calls to
check.softErrorf. Both check.versionErrorf and check.softErrorf call
check.err(at, <string>, true) after massaging the string message.

Fixes #51531

Change-Id: If54e179f5952b97701d1dfde4abb08101de07811
GitHub-Last-Rev: b0b7c1346f3a92f70e6cd5ff9ef047f441b09895
GitHub-Pull-Request: golang/go#51536
Reviewed-on: https://go-review.googlesource.com/c/go/+/390578
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Findley <rfindley@google.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/test/fixedbugs/issue51531.go b/test/fixedbugs/issue51531.go
new file mode 100644
index 0000000000..a296bbc776
--- /dev/null
+++ b/test/fixedbugs/issue51531.go
@@ -0,0 +1,13 @@
+// errorcheck -lang=go1.17
+
+// 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
+
+type empty interface{}
+
+type Foo[T empty] int // ERROR "type parameter requires go1\.18 or later \(-lang was set to go1\.17; check go.mod\)"
+
+func Bar[T empty]() {} // ERROR "type parameter requires go1\.18 or later \(-lang was set to go1\.17; check go.mod\)"