diff --git a/src/cmd/compile/internal/noder/noder.go b/src/cmd/compile/internal/noder/noder.go
index 2b67a91b3f7..e1b485b2b35 100644
--- a/src/cmd/compile/internal/noder/noder.go
+++ b/src/cmd/compile/internal/noder/noder.go
@@ -35,7 +35,7 @@ func LoadPackage(filenames []string) {
 	supportsGenerics := base.Flag.G != 0 || buildcfg.Experiment.Unified
 
 	mode := syntax.CheckBranches
-	if supportsGenerics && types.AllowsGoVersion(types.LocalPkg, 1, 18) {
+	if supportsGenerics {
 		mode |= syntax.AllowGenerics
 	}
 
diff --git a/src/cmd/compile/internal/types2/call.go b/src/cmd/compile/internal/types2/call.go
index 4bbc524856a..0b062b4c94d 100644
--- a/src/cmd/compile/internal/types2/call.go
+++ b/src/cmd/compile/internal/types2/call.go
@@ -15,6 +15,10 @@ import (
 // funcInst type-checks a function instantiation inst and returns the result in x.
 // The operand x must be the evaluation of inst.X and its type must be a signature.
 func (check *Checker) funcInst(x *operand, inst *syntax.IndexExpr) {
+	if !check.allowVersion(check.pkg, 1, 18) {
+		check.softErrorf(inst.Pos(), "function instantiation requires go1.18 or later")
+	}
+
 	xlist := unpackExpr(inst.Index)
 	targs := check.typeList(xlist)
 	if targs == nil {
@@ -318,6 +322,13 @@ func (check *Checker) arguments(call *syntax.CallExpr, sig *Signature, targs []T
 
 	// infer type arguments and instantiate signature if necessary
 	if sig.TParams().Len() > 0 {
+		if !check.allowVersion(check.pkg, 1, 18) {
+			if iexpr, _ := call.Fun.(*syntax.IndexExpr); iexpr != nil {
+				check.softErrorf(iexpr.Pos(), "function instantiation requires go1.18 or later")
+			} else {
+				check.softErrorf(call.Pos(), "implicit function instantiation requires go1.18 or later")
+			}
+		}
 		// TODO(gri) provide position information for targs so we can feed
 		//           it to the instantiate call for better error reporting
 		targs := check.infer(call.Pos(), sig.TParams().list(), targs, sigParams, args, true)
diff --git a/src/cmd/compile/internal/types2/decl.go b/src/cmd/compile/internal/types2/decl.go
index 342e1090de5..d7a33546aa4 100644
--- a/src/cmd/compile/internal/types2/decl.go
+++ b/src/cmd/compile/internal/types2/decl.go
@@ -514,11 +514,26 @@ func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init syntax.Expr) {
 	check.initVars(lhs, []syntax.Expr{init}, nopos)
 }
 
+// isImportedConstraint reports whether typ is an imported type constraint.
+func (check *Checker) isImportedConstraint(typ Type) bool {
+	named, _ := typ.(*Named)
+	if named == nil || named.obj.pkg == check.pkg || named.obj.pkg == nil {
+		return false
+	}
+	u, _ := named.under().(*Interface)
+	return u != nil && u.IsConstraint()
+}
+
 func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named) {
 	assert(obj.typ == nil)
 
+	var rhs Type
 	check.later(func() {
 		check.validType(obj.typ, nil)
+		// If typ is local, an error was already reported where typ is specified/defined.
+		if check.isImportedConstraint(rhs) && !check.allowVersion(check.pkg, 1, 18) {
+			check.errorf(tdecl.Type.Pos(), "using type constraint %s requires go1.18 or later", rhs)
+		}
 	})
 
 	alias := tdecl.Alias
@@ -540,7 +555,8 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named
 		}
 
 		obj.typ = Typ[Invalid]
-		obj.typ = check.anyType(tdecl.Type)
+		rhs = check.anyType(tdecl.Type)
+		obj.typ = rhs
 		return
 	}
 
@@ -555,8 +571,9 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named
 	}
 
 	// determine underlying type of named
-	named.fromRHS = check.definedType(tdecl.Type, named)
-	assert(named.fromRHS != nil)
+	rhs = check.definedType(tdecl.Type, named)
+	assert(rhs != nil)
+	named.fromRHS = rhs
 	// The underlying type of named may be itself a named type that is
 	// incomplete:
 	//
diff --git a/src/cmd/compile/internal/types2/resolver.go b/src/cmd/compile/internal/types2/resolver.go
index 018a20cfb2d..34fbc3d41b4 100644
--- a/src/cmd/compile/internal/types2/resolver.go
+++ b/src/cmd/compile/internal/types2/resolver.go
@@ -412,52 +412,60 @@ func (check *Checker) collectObjects() {
 				}
 
 			case *syntax.TypeDecl:
+				if len(s.TParamList) != 0 && !check.allowVersion(pkg, 1, 18) {
+					check.softErrorf(s.TParamList[0], "type parameters require go1.18 or later")
+				}
 				obj := NewTypeName(s.Name.Pos(), pkg, s.Name.Value, nil)
 				check.declarePkgObj(s.Name, obj, &declInfo{file: fileScope, tdecl: s})
 
 			case *syntax.FuncDecl:
-				d := s // TODO(gri) get rid of this
-				name := d.Name.Value
-				obj := NewFunc(d.Name.Pos(), pkg, name, nil)
-				if d.Recv == nil {
+				name := s.Name.Value
+				obj := NewFunc(s.Name.Pos(), pkg, name, nil)
+				hasTParamError := false // avoid duplicate type parameter errors
+				if s.Recv == nil {
 					// regular function
 					if name == "init" || name == "main" && pkg.name == "main" {
-						if d.TParamList != nil {
-							check.softErrorf(d, "func %s must have no type parameters", name)
+						if len(s.TParamList) != 0 {
+							check.softErrorf(s.TParamList[0], "func %s must have no type parameters", name)
+							hasTParamError = true
 						}
-						if t := d.Type; len(t.ParamList) != 0 || len(t.ResultList) != 0 {
-							check.softErrorf(d, "func %s must have no arguments and no return values", name)
+						if t := s.Type; len(t.ParamList) != 0 || len(t.ResultList) != 0 {
+							check.softErrorf(s, "func %s must have no arguments and no return values", name)
 						}
 					}
 					// don't declare init functions in the package scope - they are invisible
 					if name == "init" {
 						obj.parent = pkg.scope
-						check.recordDef(d.Name, obj)
+						check.recordDef(s.Name, obj)
 						// init functions must have a body
-						if d.Body == nil {
+						if s.Body == nil {
 							// TODO(gri) make this error message consistent with the others above
 							check.softErrorf(obj.pos, "missing function body")
 						}
 					} else {
-						check.declare(pkg.scope, d.Name, obj, nopos)
+						check.declare(pkg.scope, s.Name, obj, nopos)
 					}
 				} else {
 					// method
 					// d.Recv != nil
-					if !acceptMethodTypeParams && len(d.TParamList) != 0 {
+					if !acceptMethodTypeParams && len(s.TParamList) != 0 {
 						//check.error(d.TParamList.Pos(), invalidAST + "method must have no type parameters")
-						check.error(d, invalidAST+"method must have no type parameters")
+						check.error(s.TParamList[0], invalidAST+"method must have no type parameters")
+						hasTParamError = true
 					}
-					ptr, recv, _ := check.unpackRecv(d.Recv.Type, false)
+					ptr, recv, _ := check.unpackRecv(s.Recv.Type, false)
 					// (Methods with invalid receiver cannot be associated to a type, and
 					// methods with blank _ names are never found; no need to collect any
 					// of them. They will still be type-checked with all the other functions.)
 					if recv != nil && name != "_" {
 						methods = append(methods, methodInfo{obj, ptr, recv})
 					}
-					check.recordDef(d.Name, obj)
+					check.recordDef(s.Name, obj)
 				}
-				info := &declInfo{file: fileScope, fdecl: d}
+				if len(s.TParamList) != 0 && !check.allowVersion(pkg, 1, 18) && !hasTParamError {
+					check.softErrorf(s.TParamList[0], "type parameters require go1.18 or later")
+				}
+				info := &declInfo{file: fileScope, fdecl: s}
 				// Methods are not package-level objects but we still track them in the
 				// object map so that we can handle them like regular functions (if the
 				// receiver is invalid); also we need their fdecl info when associating
diff --git a/src/cmd/compile/internal/types2/subst.go b/src/cmd/compile/internal/types2/subst.go
index 918e5f3043b..ff8dd13b6d7 100644
--- a/src/cmd/compile/internal/types2/subst.go
+++ b/src/cmd/compile/internal/types2/subst.go
@@ -38,7 +38,7 @@ func (m substMap) lookup(tpar *TypeParam) Type {
 // subst returns the type typ with its type parameters tpars replaced by the
 // corresponding type arguments targs, recursively. subst doesn't modify the
 // incoming type. If a substitution took place, the result type is different
-// from from the incoming type.
+// from the incoming type.
 //
 // If the given typMap is non-nil, it is used in lieu of check.typMap.
 func (check *Checker) subst(pos syntax.Pos, typ Type, smap substMap, typMap map[string]*Named) Type {
diff --git a/src/cmd/compile/internal/types2/testdata/check/decls0.src b/src/cmd/compile/internal/types2/testdata/check/decls0.src
index f051a4f2acb..09e5d5c5ad8 100644
--- a/src/cmd/compile/internal/types2/testdata/check/decls0.src
+++ b/src/cmd/compile/internal/types2/testdata/check/decls0.src
@@ -146,7 +146,7 @@ type (
 		m1(I5)
 	}
 	I6 interface {
-		S0 /* ERROR "not an interface" */
+		S0 /* ERROR "non-interface type S0" */
 	}
 	I7 interface {
 		I1
diff --git a/src/cmd/compile/internal/types2/testdata/check/issues.src b/src/cmd/compile/internal/types2/testdata/check/issues.src
index 692ed37ef4b..d83a95af0ef 100644
--- a/src/cmd/compile/internal/types2/testdata/check/issues.src
+++ b/src/cmd/compile/internal/types2/testdata/check/issues.src
@@ -79,11 +79,11 @@ func issue9473(a []int, b ...int) {
 // Check that embedding a non-interface type in an interface results in a good error message.
 func issue10979() {
 	type _ interface {
-		int /* ERROR int is not an interface */
+		int /* ERROR non-interface type int */
 	}
 	type T struct{}
 	type _ interface {
-		T /* ERROR T is not an interface */
+		T /* ERROR non-interface type T */
 	}
 	type _ interface {
 		nosuchtype /* ERROR undeclared name: nosuchtype */
@@ -280,7 +280,7 @@ type issue25301b /* ERROR cycle */ = interface {
 }
 
 type issue25301c interface {
-	notE // ERROR struct\{\} is not an interface
+	notE // ERROR non-interface type struct\{\}
 }
 
 type notE = struct{}
diff --git a/src/cmd/compile/internal/types2/testdata/check/main.go2 b/src/cmd/compile/internal/types2/testdata/check/main.go2
index b7ddeaa1a88..395e3bfec80 100644
--- a/src/cmd/compile/internal/types2/testdata/check/main.go2
+++ b/src/cmd/compile/internal/types2/testdata/check/main.go2
@@ -4,4 +4,4 @@
 
 package main
 
-func /* ERROR "func main must have no type parameters" */ main[T any]() {}
+func main [T /* ERROR "func main must have no type parameters" */ any]() {}
diff --git a/src/cmd/compile/internal/types2/testdata/check/typeparams.go2 b/src/cmd/compile/internal/types2/testdata/check/typeparams.go2
index 1ad80b1e1b9..765d561f3b5 100644
--- a/src/cmd/compile/internal/types2/testdata/check/typeparams.go2
+++ b/src/cmd/compile/internal/types2/testdata/check/typeparams.go2
@@ -304,8 +304,8 @@ var _ = f8[int, float64](0, 0, nil...) // test case for #18268
 // init functions cannot have type parameters
 
 func init() {}
-func init[/* ERROR func init must have no type parameters */ _ any]() {}
-func init[/* ERROR func init must have no type parameters */ P any]() {}
+func init[_ /* ERROR func init must have no type parameters */ any]() {}
+func init[P /* ERROR func init must have no type parameters */ any]() {}
 
 type T struct {}
 
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47818.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47818.go2
new file mode 100644
index 00000000000..5334695b5e9
--- /dev/null
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47818.go2
@@ -0,0 +1,59 @@
+// 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.
+
+// Parser accepts type parameters but the type checker
+// needs to report any operations that are not permitted
+// before Go 1.18.
+
+package go1_17
+
+type T[P /* ERROR type parameters require go1\.18 or later */ any] struct{}
+
+// for init (and main, but we're not in package main) we should only get one error
+func init[P /* ERROR func init must have no type parameters */ any]()   {}
+func main[P /* ERROR type parameters require go1\.18 or later */ any]() {}
+
+func f[P /* ERROR type parameters require go1\.18 or later */ any](x P) {
+	var _ T[ /* ERROR type instantiation requires go1\.18 or later */ int]
+	var _ (T[ /* ERROR type instantiation requires go1\.18 or later */ int])
+	_ = T[ /* ERROR type instantiation requires go1\.18 or later */ int]{}
+	_ = T[ /* ERROR type instantiation requires go1\.18 or later */ int](struct{}{})
+}
+
+func (T[ /* ERROR type instantiation requires go1\.18 or later */ P]) g(x int) {
+	f[ /* ERROR function instantiation requires go1\.18 or later */ int](0)     // explicit instantiation
+	(f[ /* ERROR function instantiation requires go1\.18 or later */ int])(0)   // parentheses (different code path)
+	f( /* ERROR implicit function instantiation requires go1\.18 or later */ x) // implicit instantiation
+}
+
+type C1 interface {
+	comparable // ERROR undeclared name: comparable \(requires version go1\.18 or later\)
+}
+
+type C2 interface {
+	comparable // ERROR undeclared name: comparable \(requires version go1\.18 or later\)
+	int        // ERROR embedding non-interface type int requires go1\.18 or later
+	~ /* ERROR embedding interface element ~int requires go1\.18 or later */ int
+	int /* ERROR embedding interface element int\|~string requires go1\.18 or later */ | ~string
+}
+
+type _ interface {
+	// errors for these were reported with their declaration
+	C1
+	C2
+}
+
+type (
+	_ comparable // ERROR undeclared name: comparable \(requires version go1\.18 or later\)
+	// errors for these were reported with their declaration
+	_ C1
+	_ C2
+
+	_ = comparable // ERROR undeclared name: comparable \(requires version go1\.18 or later\)
+	// errors for these were reported with their declaration
+	_ = C1
+	_ = C2
+)
+
+// TODO(gri) need test cases for imported constraint types (see also issue #47967)
\ No newline at end of file
diff --git a/src/cmd/compile/internal/types2/typeset.go b/src/cmd/compile/internal/types2/typeset.go
index 14596b68a30..56f64ab4059 100644
--- a/src/cmd/compile/internal/types2/typeset.go
+++ b/src/cmd/compile/internal/types2/typeset.go
@@ -271,6 +271,11 @@ func computeInterfaceTypeSet(check *Checker, pos syntax.Pos, ityp *Interface) *_
 		switch u := under(typ).(type) {
 		case *Interface:
 			tset := computeInterfaceTypeSet(check, pos, u)
+			// If typ is local, an error was already reported where typ is specified/defined.
+			if check != nil && check.isImportedConstraint(typ) && !check.allowVersion(check.pkg, 1, 18) {
+				check.errorf(pos, "embedding constraint interface %s requires go1.18 or later", typ)
+				continue
+			}
 			if tset.comparable {
 				ityp.tset.comparable = true
 			}
@@ -279,6 +284,10 @@ func computeInterfaceTypeSet(check *Checker, pos syntax.Pos, ityp *Interface) *_
 			}
 			terms = tset.terms
 		case *Union:
+			if check != nil && !check.allowVersion(check.pkg, 1, 18) {
+				check.errorf(pos, "embedding interface element %s requires go1.18 or later", u)
+				continue
+			}
 			tset := computeUnionTypeSet(check, pos, u)
 			if tset == &invalidTypeSet {
 				continue // ignore invalid unions
@@ -293,7 +302,7 @@ func computeInterfaceTypeSet(check *Checker, pos syntax.Pos, ityp *Interface) *_
 				continue
 			}
 			if check != nil && !check.allowVersion(check.pkg, 1, 18) {
-				check.errorf(pos, "%s is not an interface", typ)
+				check.errorf(pos, "embedding non-interface type %s requires go1.18 or later", typ)
 				continue
 			}
 			terms = termlist{{false, typ}}
diff --git a/src/cmd/compile/internal/types2/typexpr.go b/src/cmd/compile/internal/types2/typexpr.go
index 241c6d35fe4..f3e415e4c73 100644
--- a/src/cmd/compile/internal/types2/typexpr.go
+++ b/src/cmd/compile/internal/types2/typexpr.go
@@ -38,14 +38,12 @@ func (check *Checker) ident(x *operand, e *syntax.Name, def *Named, wantType boo
 		}
 		return
 	case universeAny, universeComparable:
+		// complain if necessary but keep going
 		if !check.allowVersion(check.pkg, 1, 18) {
-			check.errorf(e, "undeclared name: %s (requires version go1.18 or later)", e.Value)
-			return
-		}
-		// If we allow "any" for general use, this if-statement can be removed (issue #33232).
-		if obj == universeAny {
-			check.error(e, "cannot use any outside constraint position")
-			return
+			check.softErrorf(e, "undeclared name: %s (requires version go1.18 or later)", e.Value)
+		} else if obj == universeAny {
+			// If we allow "any" for general use, this if-statement can be removed (issue #33232).
+			check.softErrorf(e, "cannot use any outside constraint position")
 		}
 	}
 	check.recordUse(e, obj)
@@ -274,6 +272,9 @@ func (check *Checker) typInternal(e0 syntax.Expr, def *Named) (T Type) {
 		}
 
 	case *syntax.IndexExpr:
+		if !check.allowVersion(check.pkg, 1, 18) {
+			check.softErrorf(e.Pos(), "type instantiation requires go1.18 or later")
+		}
 		return check.instantiatedType(e.X, unpackExpr(e.Index), def)
 
 	case *syntax.ParenExpr:
diff --git a/test/fixedbugs/issue10975.go b/test/fixedbugs/issue10975.go
index 876ea58ef97..a58ccce2db1 100644
--- a/test/fixedbugs/issue10975.go
+++ b/test/fixedbugs/issue10975.go
@@ -10,7 +10,7 @@
 package main
 
 type I interface {
-	int // ERROR "interface contains embedded non-interface|not an interface"
+	int // ERROR "interface contains embedded non-interface|embedding non-interface type"
 }
 
 func New() I {