Fix #4737

tests/errmsgs/tconceptconstraint.nim

@@ -0,0 +1,21 @@
+discard """
+  errormsg: "cannot instantiate B"
+  line: 20
+  nimout: '''
+got: (type string)
+but expected: (T: A)
+'''
+"""
+
+type
+  A = concept c
+    advance(c)
+  
+  B[T: A] = object
+    child: ref B[T]
+
+proc advance(x: int): int = x + 1
+
+var a: B[int]
+var b: B[string]
+

tests/errmsgs/tgenericconstraint.nim

@@ -0,0 +1,15 @@
+discard """
+  errormsg: "cannot instantiate B"
+  line: 14
+  nimout: '''
+got: (type int)
+but expected: (T: string or float)
+'''
+"""
+
+type
+  B[T: string|float] = object
+    child: ref B[T]
+
+var b: B[int]
+