proper error reporting for concepts and the introduction of the {.explain.} pragma

tests/concepts/texplain.nim

@@ -0,0 +1,92 @@
+discard """
+  cmd: "nim c --verbosity:0 --colors:off $file"
+  nimout: '''
+tests/concepts/texplain.nim(71, 10) Hint: Non-matching candidates for e(y)
+proc e(i: int): int
+ [User]
+tests/concepts/texplain.nim(74, 7) Hint: Non-matching candidates for e(10)
+proc e[ExplainedConcept](o: ExplainedConcept): int
+tests/concepts/texplain.nim(38, 6) Error: undeclared field: 'foo'
+tests/concepts/texplain.nim(38, 6) Error: undeclared field: '.'
+tests/concepts/texplain.nim(38, 6) Error: type mismatch: got (
+ [User]
+tests/concepts/texplain.nim(77, 10) Hint: Non-matching candidates for e(10)
+proc e[ExplainedConcept](o: ExplainedConcept): int
+tests/concepts/texplain.nim(38, 6) Error: undeclared field: 'foo'
+tests/concepts/texplain.nim(38, 6) Error: undeclared field: '.'
+tests/concepts/texplain.nim(38, 6) Error: type mismatch: got (
+ [User]
+tests/concepts/texplain.nim(81, 20) Error: type mismatch: got (
+tests/concepts/texplain.nim(82, 20) Error: type mismatch: got (
+tests/concepts/texplain.nim(83, 20) Hint: Non-matching candidates for r(y)
+proc r(i: string): int
+ [User]
+tests/concepts/texplain.nim(91, 2) Error: type mismatch: got (MatchingType)
+but expected one of: 
+proc f[NestedConcept](o: NestedConcept)
+tests/concepts/texplain.nim(42, 6) Error: undeclared field: 'foo'
+tests/concepts/texplain.nim(42, 6) Error: undeclared field: '.'
+tests/concepts/texplain.nim(42, 6) Error: type mismatch: got (
+tests/concepts/texplain.nim(46, 5) Error: type class predicate failed
+'''
+  line: 46
+  errormsg: "type class predicate failed"
+"""
+
+type
+  ExplainedConcept {.explain.} = concept o
+    o.foo is int
+    o.bar is string
+
+  RegularConcept = concept o
+    o.foo is int
+    o.bar is string
+
+  NestedConcept = concept o
+    o.foo is RegularConcept
+
+  NonMatchingType = object
+    foo: int
+    bar: int
+
+  MatchingType = object
+    foo: int
+    bar: string
+
+proc e(o: ExplainedConcept): int = 1
+proc e(i: int): int = i
+
+proc r(o: RegularConcept): int = 1
+proc r(i: string): int = 1
+
+proc f(o: NestedConcept) = discard
+
+var n = NonMatchingType(foo: 10, bar: 20)
+var y = MatchingType(foo: 10, bar: "bar")
+
+# no diagnostic here:
+discard e(y)
+
+# explain that e(int) doesn't match
+discard e(y) {.explain.}
+
+# explain that e(ExplainedConcept) doesn't match
+echo(e(10) {.explain.}, 20)
+
+# explain that e(ExplainedConcept) doesn't again
+discard e(10)
+
+static:
+  # provide diagnostics why the compile block failed 
+  assert(compiles(e(n)) {.explain.} == false)
+  assert(compiles(r(n)) {.explain.} == false)
+  assert(compiles(r(y)) {.explain.} == true)
+  
+  # these should not produce any output
+  assert(compiles(r(10)) == false)
+  assert(compiles(e(10)) == true)
+
+# finally, provide multiple nested explanations for failed matching
+# of regular concepts, even when the explain pragma is not used
+f(y)
+