go/types, types2: factor out single commonUnder function

Combine commonUnder and commonUnderOrChan:
- Provide an optional cond(ition) function argument to commonUnder
  to establish additional type set conditions.
- Instead of a *Checker and *string argument for error reporting,
  return an error cause that is only allocated in the presence of
  an error.
- Streamline some error messages.

Replace all calls to coreType with calls to commonUnder.

Change-Id: I81ac86d0d532cddc09164309acced61d90718b44
Reviewed-on: https://go-review.googlesource.com/c/go/+/654455
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

2 files changed, 8 insertions, 8 deletions

diff --git a/src/internal/types/testdata/fixedbugs/issue43671.go b/src/internal/types/testdata/fixedbugs/issue43671.go
index 19da7e0ccc..5b44682a7a 100644
--- a/src/internal/types/testdata/fixedbugs/issue43671.go
+++ b/src/internal/types/testdata/fixedbugs/issue43671.go
@@ -12,11 +12,11 @@ type C4 interface{ chan int | chan<- int }
 type C5[T any] interface{ ~chan T | <-chan T }
 
 func _[T any](ch T) {
-	<-ch // ERRORx `cannot receive from ch .*: type set contains no specific channel type`
+	<-ch // ERRORx `cannot receive from ch .*: no specific channel type`
 }
 
 func _[T C0](ch T) {
-	<-ch // ERRORx `cannot receive from ch .*: type set contains non-channel int`
+	<-ch // ERRORx `cannot receive from ch .*: non-channel int`
 }
 
 func _[T C1](ch T) {
@@ -28,11 +28,11 @@ func _[T C2](ch T) {
 }
 
 func _[T C3](ch T) {
-	<-ch // ERRORx `cannot receive from ch .*: type set contains channels with different element types int and float32`
+	<-ch // ERRORx `cannot receive from ch .*: channels chan int and chan float32 have different element types`
 }
 
 func _[T C4](ch T) {
-	<-ch // ERRORx `cannot receive from ch .*: type set contains send-only channel chan<- int`
+	<-ch // ERRORx `cannot receive from ch .*: send-only channel chan<- int`
 }
 
 func _[T C5[X], X any](ch T, x X) {
diff --git a/src/internal/types/testdata/fixedbugs/issue47115.go b/src/internal/types/testdata/fixedbugs/issue47115.go
index 1de85b3791..58b668ce4f 100644
--- a/src/internal/types/testdata/fixedbugs/issue47115.go
+++ b/src/internal/types/testdata/fixedbugs/issue47115.go
@@ -12,11 +12,11 @@ type C4 interface{ chan int | chan<- int }
 type C5[T any] interface{ ~chan T | chan<- T }
 
 func _[T any](ch T) {
-	ch <- /* ERRORx `cannot send to ch .*: type set contains no specific channel type` */ 0
+	ch <- /* ERRORx `cannot send to ch .*: no specific channel type` */ 0
 }
 
 func _[T C0](ch T) {
-	ch <- /* ERRORx `cannot send to ch .*: type set contains non-channel int` */ 0
+	ch <- /* ERRORx `cannot send to ch .*: non-channel int` */ 0
 }
 
 func _[T C1](ch T) {
@@ -24,11 +24,11 @@ func _[T C1](ch T) {
 }
 
 func _[T C2](ch T) {
-	ch <- /* ERRORx `cannot send to ch .*: type set contains receive-only channel <-chan int` */ 0
+	ch <- /* ERRORx `cannot send to ch .*: receive-only channel <-chan int` */ 0
 }
 
 func _[T C3](ch T) {
-	ch <- /* ERRORx `cannot send to ch .*: type set contains channels with different element types` */ 0
+	ch <- /* ERRORx `cannot send to ch .*: channels chan int and chan float32 have different element types` */ 0
 }
 
 func _[T C4](ch T) {